groff_mdoc(7): Revise built-in string discussion.

* Stop referring to table column heading as a literal.  Mundane
  quotation suffices.
* Offer cross reference to groff_char(7), since it is authoritative,
  comprehensive, and we discourage the use of the strings shown here.
* Tweak table format to center "String" heading, due to content change
  below.  Set string and special character columns in Courier on
  typesetting devices.  Capitalize column classifiers as suggested in
  tbl(1).
* Use string interpolation notation in "Strings" column.
* Condense background about glyph substitution, also eliding an error
  that goes back at least to commit b57a73283, 2001-03-24; there is a
  second relevant difference between ASCII and Latin-1 repertoires: the
  acute accent.
* Drop now-pointless presentation of string interpolation syntax.  As
  much as the mdoc(7) page author needs to know can be read out of the
  table, and we're deprecating that in favor of special characters
  anyway.

1 files changed, 50 insertions, 60 deletions

diff --git a/tmac/groff_mdoc.7.man b/tmac/groff_mdoc.7.man
index 48d283ddb..e9c6cfea6 100644
--- a/tmac/groff_mdoc.7.man
+++ b/tmac/groff_mdoc.7.man
@@ -4666,9 +4666,13 @@ The following strings are predefined for compatibility with legacy
 documents.
 .
 Contemporary ones should use the alternatives shown in the
-.Ql Prefer
+.Dq Prefer
 column below.
 .
+See
+.Xr groff_char @MAN7EXT@
+for a full discussion of these special character escape sequences.
+.
 .
 .Pp
 .\" Note: This table pushes 80-column ASCII and Latin-1 terminals to
@@ -4678,51 +4682,46 @@ column below.
 .\" written out in full, which in turn pushes the >= and Ge rows past
 .\" the 78n norm.  Update with caution.  See Savannah #59424.
 .TS
-lb lb2 lb2 lb lb lb
-l l2 l2 l l l.
+Cb Lb2 Lb2 Lb Lb Lb
+Lf(CR) L2 L2 L Lf(CR) L.
 String	7-bit	8-bit	UCS	Prefer	Meaning
-<=	<=  	<=	\*[<=]	\[rs](<=	less than or equal to
->=	>=  	>=	\*[>=]	\[rs](>=	greater than or equal to
-Rq	"  	"	\*[Rq]	\[rs](rq	right double quote
-Lq	"  	"	\*[Lq]	\[rs](lq	left double quote
-ua	\[ha]	\[ha]	\*[ua]	\[rs](ua	vertical arrow up
-aa	\[aq]	\'	\*[aa]	\[rs](aa	acute accent
-ga	\`  	\`	\*[ga]	\[rs](ga	grave accent
-q 	\&" 	\&"	\*[q] 	\[rs](dq	neutral double quote
-Pi	pi  	pi	\*[Pi]	\[rs](*p	lowercase pi
-Ne	!=  	!=	\*[Ne]	\[rs](!=	not equals
-Le	<=  	<=	\*[Le]	\[rs](<=	less than or equal to
-Ge	>=  	>=	\*[Ge]	\[rs](>=	greater than or equal to
-Lt	<   	<	\*[Lt]	<	less than
-Gt	>   	>	\*[Gt]	>	greater than
-Pm	+\- 	\[+-]	\*[Pm]	\[rs](+\-	plus or minus
-If	infinity	infinity	\*[If]	\[rs](if	infinity
-Am	\*[Am]	\*[Am]	\*[Am]	&	ampersand
-Na	\*[Na]	\*[Na]	\*[Na]	NaN	not a number
-Ba	\*[Ba]	\*[Ba]	\*[Ba]	|	bar
+\[rs]*(<=	<=  	<=	\*[<=]	\[rs](<=	less than or equal to
+\[rs]*(>=	>=  	>=	\*[>=]	\[rs](>=	greater than or equal to
+\[rs]*(Rq	"  	"	\*[Rq]	\[rs](rq	right double quote
+\[rs]*(Lq	"  	"	\*[Lq]	\[rs](lq	left double quote
+\[rs]*(ua	\[ha]	\[ha]	\*[ua]	\[rs](ua	vertical arrow up
+\[rs]*(aa	\[aq]	\'	\*[aa]	\[rs](aa	acute accent
+\[rs]*(ga	\`  	\`	\*[ga]	\[rs](ga	grave accent
+\[rs]*(q 	\&" 	\&"	\*[q] 	\[rs](dq	neutral double quote
+\[rs]*(Pi	pi  	pi	\*[Pi]	\[rs](*p	lowercase pi
+\[rs]*(Ne	!=  	!=	\*[Ne]	\[rs](!=	not equals
+\[rs]*(Le	<=  	<=	\*[Le]	\[rs](<=	less than or equal to
+\[rs]*(Ge	>=  	>=	\*[Ge]	\[rs](>=	greater than or equal to
+\[rs]*(Lt	<   	<	\*[Lt]	<	less than
+\[rs]*(Gt	>   	>	\*[Gt]	>	greater than
+\[rs]*(Pm	+\- 	\[+-]	\*[Pm]	\[rs](+\-	plus or minus
+\[rs]*(If	infinity	infinity	\*[If]	\[rs](if	infinity
+\[rs]*(Am	\*[Am]	\*[Am]	\*[Am]	&	ampersand
+\[rs]*(Na	\*[Na]	\*[Na]	\*[Na]	NaN	not a number
+\[rs]*(Ba	\*[Ba]	\*[Ba]	\*[Ba]	|	bar
 .TE
 .
 .
 .Pp
-The names of the columns are shorthand for standard character encodings;
+Some column headings are shorthand for standardized character encodings;
 \[lq]7-bit\[rq] for ISO 646:1991 IRV (US-ASCII),
 \[lq]8-bit\[rq] for ISO 8859-1 (Latin-1) and IBM code page 1047,
 and
 \[lq]UCS\[rq] for ISO 10646 (Unicode character set).
 .
-The strings were originally defined
-(circa 1989)
-such that the best available representation would be used.
+Historically, \" (\[ti]1989)
+.Xr mdoc
+configured the string definitions to fit the capabilities expected of
+the the output device.
 .
-For example,
-a Unicode-enabled terminal device using a well-chosen font will have
-optimal glyph representations for all of the above,
-whereas a Latin-1 terminal's only advantage over an ASCII one
-(for these glyphs)
-is the plus-minus sign.
-.
-Old typesetters lacked directional double quotes, \" like the C/A/T
-and would produce repeated directional single quotes,
+Old typesetters \" like the C/A/T
+lacked directional double quotes,
+producing repeated directional single quotes
 \[oq]\[oq]like this\[cq]\[cq];
 early versions of
 .Xr mdoc
@@ -4732,44 +4731,35 @@ and
 .Ql Rq
 strings this way. \" thanks to Ingo Schwarze for the research
 .
-In the years since,
-output drivers have taken on the responsibility of glyph substitution;
-they are the part of the formatting process with relevant knowledge of
-the available repertoire.
-.
-.
-.Pp
-String names which consist of two characters can be written as
-.Ql \[rs]*(xx ;
-string names which consist of one character can be written as
-.Ql \[rs]*x .
-A general syntax for a string name of any length is
-.Ql \[rs]*[xxx]
-(this is a
-.Tn GNU
-.Xr @g@troff @MAN1EXT@
-extension).
+Nowadays,
+output drivers \" technically, their macro files or font descriptions
+take on the responsibility of glyph substitution,
+as they possess relevant knowledge of their available repertoires.
 .
 .
 .Sh Diagnostics
 .
 The debugging macro
 .Ql .Db
-available in previous versions of
+offered by
+previous versions of
 .Xr mdoc
-has been removed since
+is unavailable in
 .Tn GNU
 .Xr @g@troff @MAN1EXT@
-provides better facilities to check parameters;
+since the latter provides better facilities to check parameters;
 additionally,
-many error and warning messages have been added to this macro package,
-making it both more robust and verbose.
+.Xr "groff mdoc"
+implements many error and warning messages,
+making the package more robust and more verbose.
 .
 .
 .Pp
-The only remaining debugging macro is
-.Ql .Rd
-which yields a register dump of all global registers and strings.
+The remaining debugging macro is
+.Ql .Rd ,
+which dumps the package's global register and string contents to the
+standard error stream.
+.
 A normal user will never need it.
 .
 .