Radical cleanup of COMPATIBILITY sections:

Remove lots of lies, dozens of irrelevant implementation details,
and all references to groff versions older than 1.17.  Move relevant
information to the pages where it belongs, and out of mandoc(1) in
particular.  Add some missing general remarks to roff(7), where it
fits the character and purpose of the page much better.

1 files changed, 46 insertions, 30 deletions

diff --git a/roff.7 b/roff.7
index efe53e74..81413919 100644
--- a/roff.7
+++ b/roff.7
@@ -2028,50 +2028,66 @@ approximated in
 .Xr mandoc 1
 by simply skipping the next character.
 .Sh COMPATIBILITY
-This section documents compatibility between mandoc and other
+The
+.Xr mandoc 1
+implementation of the
 .Nm
-implementations, at this time limited to GNU troff
-.Pq Qq groff .
-The term
-.Qq historic groff
-refers to groff version 1.15.
+language is intentionally incomplete.
+Unimplemented features include:
 .Pp
 .Bl -dash -compact
 .It
+For security reasons,
+.Xr mandoc 1
+never reads or writes external files except via
+.Sx \&so
+requests with safe relative paths.
+.It
+There is no automatic hyphenation, no adjustment to the right margin,
+and no centering; the output is always set flush-left.
+.It
+Support for setting tabulator positions
+and tabulator and leader characters is missing,
+and support for manually changing indentation is limited.
+.It
 The
 .Sq u
 scaling unit is the default terminal unit.
-In traditional troff systems, this unit would change depending on the
+In traditional troff systems, this unit changes depending on the
 output media.
 .It
-In mandoc, the
-.Sx \&EQ ,
-.Sx \&TE ,
-.Sx \&TS ,
-and
-.Sx \&T& ,
-macros are considered regular macros.
-In all other
-.Nm
-implementations, these are special macros that must be specified without
-spacing between the control character (which must be a period) and the
-macro name.
+Width measurements are implemented in a crude way
+and often yield wrong results.
+Explicit movement requests and escapes are ignored.
 .It
-The
-.Cm nS
-register is only compatible with OpenBSD's groff-1.15.
+There is no concept of output pages, no support for floats,
+graphics drawing, and picture inclusion;
+terminal output is always continuous.
 .It
-Historic groff did not accept white-space before a custom
-.Ar end
-macro for the
-.Sx \&ig
-request.
+Requests regarding color, font families, and glyph manipulation
+are ignored.
+Font support is very limited.
+Kerning is not implemented, and no ligatures are produced.
 .It
 The
-.Sx \&if
-and family would print funny white-spaces with historic groff when
-using the next-line syntax.
+.Qq \(aq
+macro control character does not suppress output line breaks.
+.It
+Diversions are not implemented,
+and support for traps is very incomplete.
+.It
+While recursion is supported,
+.Sx \&while
+loops are not.
 .El
+.Pp
+The special semantics of the
+.Cm nS
+number register is an idiosyncracy of
+.Ox
+manuals and not supported by other
+.Xr mdoc 7
+implementations.
 .Sh SEE ALSO
 .Xr mandoc 1 ,
 .Xr eqn 7 ,