[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.

3 files changed, 27 insertions, 19 deletions

diff --git a/doc/groff.texi b/doc/groff.texi
index a0e86a915..819bcf17b 100644
--- a/doc/groff.texi
+++ b/doc/groff.texi
@@ -8230,31 +8230,24 @@ disabled.  Omitting the argument implies an @var{nnn} of @samp{1}.
 @cindex size of sentence space register (@code{.sss})
 @cindex space between sentences register (@code{.sss})
 Set the sizes of spaces between words and
-sentences.@footnote{@xref{Filling} and @ref{Sentences} for the
-definitions of word and sentence boundaries, respectively.}  Their units
-are twelfths of the space width of the current font.  Initially both the
-@var{word-space-size} and @var{additional-sentence-space-size}
-are@tie{}12.  Negative values are not permitted.  The request is ignored
-if there are no arguments.
-
+sentences@footnote{Recall @ref{Filling} and @ref{Sentences} for the
+definitions of word and sentence boundaries, respectively.} in twelfths
+of current font's space width (usually one-third em for Western
+scripts).  Initially, both the @var{word-space-size} and
+@var{additional-sentence-space-size} are@tie{}12.  Negative values are
+erroneous.
 @cindex inter-word spacing, minimal
 @cindex minimal inter-word spacing
 @cindex space, between words
 The first argument, the inter-word space size, is a minimum; if an
 output line undergoes adjustment, such spaces may increase in width.
-
 @cindex inter-sentence space, additional
 @cindex additional inter-sentence space
 @cindex space, between sentences
 The optional second argument sets the amount of additional space
 separating sentences on the same output line.  If omitted, this amount
-is set to @var{word-space-size}.
-
-The read-only registers @code{.ss} and @code{.sss} hold the values of
-minimal inter-word space and additional inter-sentence space,
-respectively.  These parameters are associated with the environment
-(@pxref{Environments}), and rounded down to the nearest multiple
-of@tie{}12 on terminal output devices.
+is set to @var{word-space-size}.  The request is ignored if there are no
+parameters.
 
 @cindex filling, and inter-sentence space
 @cindex mode, fill, and inter-sentence space
@@ -8268,6 +8261,12 @@ inter-sentence space is not adjusted, but the inter-word space that
 always precedes it may be.  Further input spaces after the second, if
 present, are adjusted as normal.
 
+The read-only registers @code{.ss} and @code{.sss} hold the minimal
+inter-word space and additional inter-sentence space amounts,
+respectively.  These parameters are part of the environment
+(@pxref{Environments}), and rounded down to the nearest multiple
+of@tie{}12 on terminals.
+
 @cindex discardable horizontal space
 @cindex space, discardable, horizontal
 @cindex horizontal discardable space
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