[docs]: Fix Savannah #68216.

[docs]: Re-re-christen 'ESCAPE_AMPERSAND' ('\&').  Now call it a
(non-transparent) "dummy character".  Also rechristen
'ESCAPE_RIGHT_PARENTHESIS', ('\)') as the "transparent dummy character";
it has no impact on sentence-ending detection.

* doc/groff.texi:
* doc/meref.me.in:
* man/groff.7.man:
* man/groff_diff.7.man:
* man/roff.7.man:
* src/preproc/refer/refer.1.man:
* tmac/groff_man.7.man.in: Do it.

Fixes <https://savannah.gnu.org/bugs/?62816>.  Thanks to Dave Kemper for
the report and to the groff mailing list for the vigorous discussion.  I
don't expect my solution to please everyone.

Also update references to the \& escape sequence in comments.

ANNOUNCE: Update bug counts.

3 files changed, 70 insertions, 13 deletions

diff --git a/man/groff.7.man b/man/groff.7.man
index bfa93f94e..66ca0089a 100644
--- a/man/groff.7.man
+++ b/man/groff.7.man
@@ -1860,6 +1860,49 @@ See section \[lq]Implementation differences\[rq] in
 .
 .
 .\" ====================================================================
+.SH "Dummy characters"
+.\" ====================================================================
+.
+.\" BEGIN Keep (roughly) parallel with groff.texi node "Dummy
+.\" Characters".
+As discussed in
+.MR roff @MAN7EXT@ ,
+the first character on an input line is treated specially.
+.
+Further,
+formatting a glyph has many
+consequences on formatter state
+(see section \[lq]Environments\[rq] below).
+.
+Occasionally,
+we want to escape this context or embrace some of those consequences
+without actually rendering a glyph to the output.
+.
+.B \[rs]&
+interpolates a dummy character,
+which is constitutive of output but invisible.
+.
+Its presence alters the interpretation context of a subsequent input
+character,
+and enjoys several applications:
+preventing the insertion of extra space after an end-of-sentence
+character,
+preventing interpretation of a control character at the beginning of an
+input line,
+preventing kerning between two glyphs,
+and permitting the
+.B tr
+request to remap a character to nothing.
+.
+.B \[rs])
+works as
+.B \[rs]&
+does,
+except that it does not cancel a pending end-of-sentence state.
+.\" END Keep (roughly) parallel with groff.texi node "Dummy Characters".
+.
+.
+.\" ====================================================================
 .SH "Control structures"
 .\" ====================================================================
 .
@@ -4515,13 +4558,13 @@ Move one-twelfth em to the right on typesetters.
 .
 .TP
 .ESC &
-Non-printing input break.
+Interpolate a dummy character.
 .
 .
 .TP
 .ESC )
-Non-printing input break,
-transparent to end-of-sentence recognition.
+Interpolate a dummy character that is transparent to end-of-sentence
+recognition.
 .
 .
 .TP
diff --git a/man/groff_diff.7.man b/man/groff_diff.7.man
index 4ba065061..6f81cd9fe 100644
--- a/man/groff_diff.7.man
+++ b/man/groff_diff.7.man
@@ -975,14 +975,29 @@ argument to the
 .B ds
 request.
 .
+.
 .TP
 .B \[rs])
-Like
+Interpolate a
+.I transparent
+dummy character\[em]one that is transparent to end-of-sentence
+detection.
+.
+It behaves as
+.BR \[rs]& ,
+except that
 .B \[rs]&
-except that it behaves like a character declared with the
-.B .cflags
-request to be transparent for the purposes of end-of-sentence
-recognition.
+is treated as letters and numerals normally are after
+\[lq].\[rq],
+\[lq]?\[rq],
+and
+\[lq]!\[rq];
+.B \[rs]&
+cancels end-of-sentence detection,
+and
+.B \[rs])
+does not.
+.
 .
 .TP
 .BI \[rs]*[ "xxx arg1 arg2\~"\c
@@ -4442,8 +4457,7 @@ is
 If the font description file contains pairwise kerning information,
 glyphs from that font are kerned.
 .
-Kerning between two glyphs can be inhibited by placing a non-printing
-input break
+Kerning between two glyphs can be inhibited by placing a dummy character
 .B \[rs]&
 between them.
 .
diff --git a/man/roff.7.man b/man/roff.7.man
index e55f2dcdc..80831970b 100644
--- a/man/roff.7.man
+++ b/man/roff.7.man
@@ -183,7 +183,7 @@ or one of them is followed by two spaces on the same input line,
 it appends an inter-word space
 followed by an inter-sentence space in the formatted output.
 .
-The non-printing input break escape sequence
+The dummy character escape sequence
 .B \[rs]&
 can be used after an end-of-sentence character to defeat end-of-sentence
 detection on a per-instance basis.
@@ -411,8 +411,8 @@ If you require a formatted period or apostrophe
 where
 .\" GNU @code{troff}
 the formatter is expecting a control character,
-prefix the dot or neutral apostrophe with the non-printing input break
-escape sequence,
+prefix the dot or neutral apostrophe with the dummy character escape
+sequence,
 .RB \[lq] \[rs]& \[rq].
 .
 .