man/groff_diff.7.man: Update \s discussion.

1. Drop documentation of \s(nn and \s±(nn forms.  They're in CSTR #54.

2. In the \s[] and \s'' forms, set the ± in italics, not bold.  It's
   parametric, not literal, input.

3. Add paragraph to incompatibilities section regarding the handling of
   \sN and compatibility mode.

1 files changed, 45 insertions, 14 deletions

diff --git a/man/groff_diff.7.man b/man/groff_diff.7.man
index c93198197..356583064 100644
--- a/man/groff_diff.7.man
+++ b/man/groff_diff.7.man
@@ -559,23 +559,13 @@ This has the same effect as
 .RE
 .
 .TP
-.BI \[rs]s( nn
-.TQ
-.BI \[rs]s\[+-]( nn
-Set the point size to
-.I nn
-points;
-.I nn
-must be exactly two digits.
-.
-.TP
-.BI \[rs]s[\[+-] n ]
+.BI \[rs]s[ \[+-]n ]
 .TQ
-.BI \[rs]s\[+-][ n ]
+.BI \[rs]s \[+-] [ n ]
 .TQ
-.BI \[rs]s'\[+-] n '
+.BI \[rs]s' \[+-]n '
 .TQ
-.BI \[rs]s\[+-]' n '
+.BI \[rs]s \[+-] ' n '
 Set the point size to
 .I n
 scaled points;
@@ -3832,6 +3822,47 @@ sets the point size to 10\~points, whereas in groff native mode the
 point size is set to 10\~scaled points.
 .
 .P
+The escape
+.BI \[rs]s n
+sets the point size to
+.I n
+scaled points;
+.I n
+must be exactly one digit and
+is a numeric expression with a default scale indicator
+.RB of\~ z\c
+\&.
+.
+AT&T
+.I troff
+would read an additional digit,
+as will
+.I groff
+in compatibility mode,
+if
+.I n
+is 1, 2, or 3,
+because the Graphic Systems C/A/T phototypesetter
+(its original device target)
+only supported a few discrete point sizes in the range 6\[en]36,
+so Ossanna special-cased the parser to do what the user must have meant.
+.
+Kernighan warned of this in the 1992 revision of CSTR\~#54 (\[sc]2.3),
+and more recently,
+McIlroy referred to it as a \[lq]living fossil\[rq].
+.
+.I groff
+will emit an error message in compatibility mode when this ambiguous
+syntax is encountered.
+.
+Use the portable
+.BI \[rs]s( nn
+form for two-digit point sizes,
+or the new forms of
+.B \[rs]s
+documented above.
+.
+.P
 In
 .IR groff ,
 there is a fundamental difference between unformatted input