[docs]: Update discussion of `ss` request.

* doc/groff.texi (Manipulating Filling and Adjustment): Recast.
  Tighten wording.  Document conventional space width.  Fully discuss
  inter-sentence spacing behavior before documenting registers.
* man/groff_diff.7 (Extended requests): Sync with the foregoing.
* man/groff.7 (Read-only registers): Use less cramped English.

2 files changed, 13 insertions, 4 deletions

diff --git a/man/groff.7.man b/man/groff.7.man
index 13c5f5df2..c45f929e7 100644
--- a/man/groff.7.man
+++ b/man/groff.7.man
@@ -5919,13 +5919,13 @@ and
 .
 .TP
 .REG .ss
-Size of minimal inter-word spacing in twelfths of the space width of
+Size of minimal inter-word space in twelfths of the space width of the
 selected font.
 .
 .TP
 .REG .sss
 Size of additional inter-sentence space in twelfths of the space width
-of selected font.
+of the selected font.
 .
 .TP
 .REG .sty
diff --git a/man/groff_diff.7.man b/man/groff_diff.7.man
index 56770f0ef..5024e867b 100644
--- a/man/groff_diff.7.man
+++ b/man/groff_diff.7.man
@@ -3838,10 +3838,19 @@ If omitted,
 this amount is set to
 .IR word-space-size .
 .
-The arguments' units are twelfths of the space width of the current font
+Both arguments are in twelfths of current font's space width
+(usually one-third em for Western scripts;
 (see
 .MR groff_font @MAN5EXT@ )
-and default to\~12.
+.
+Initially,
+both the
+.I word-space-size
+and
+.I additional-sentence-space-size
+are\~12.
+.
+Negative values are erroneous.
 .
 .
 .TP