mandoc: Mention that .AT, .P, .SB, and .UC are extensions; it really

mandoc: Mention that .AT, .P, .SB, and .UC are extensions; it really

[schwarze]

Log Message:
-----------
Mention that .AT, .P, .SB, and .UC are extensions; it really matters
because we only retain the language for backward compatibility in
the first place.  Part of the research done by <G dot Branden dot
Robinson at gmail dot com>, see the list <groff at gnu dot org> for
details.

No change to the following conventions: Consider portable whatever
made it into GNU troff no later than 4.4BSD.  For portable extensions,
mention their origin at the end of the description.  For non-portable
extensions, for example from man-ext, usually warn earlier, near 
the beginning of the description.

Modified Files:
--------------
    mandoc:
        man.7

Revision Data
-------------
Index: man.7
===================================================================
RCS file: /home/cvs/mandoc/mandoc/man.7,v
retrieving revision 1.144
retrieving revision 1.145
diff -Lman.7 -Lman.7 -u -p -r1.144 -r1.145
--- man.7
+++ man.7
@@ -1,7 +1,7 @@
 .\"	$Id$
 .\"
 .\" Copyright (c) 2009, 2010, 2011, 2012 Kristaps Dzonsons <kristaps@bsd.lv>
-.\" Copyright (c) 2011-2015,2017,2018,2019 Ingo Schwarze <schwarze@openbsd.org>
+.\" Copyright (c) 2011-2015, 2017-2020 Ingo Schwarze <schwarze@openbsd.org>
 .\" Copyright (c) 2017 Anthony Bentley <bentley@openbsd.org>
 .\" Copyright (c) 2010 Joerg Sonnenberger <joerg@netbsd.org>
 .\"
@@ -127,6 +127,8 @@ Sets the volume for the footer for compa
 .At
 releases.
 The optional arguments specify which release it is from.
+This macro is an extension that first appeared in
+.Bx 4.3 .
 .It Ic B
 Text is rendered in bold face.
 .It Ic BI
@@ -255,8 +257,12 @@ is usually a command-line flag and
 .Ar value
 its argument.
 .It Ic P
-A synonym for
-.Ic PP .
+This synonym for
+.Ic PP
+is an
+.At III
+extension later adopted by
+.Bx 4.3 .
 .It Ic PD
 Specify the vertical space to be inserted before each new paragraph.
 .br
@@ -343,6 +349,9 @@ See also
 .It Ic SB
 Text is rendered in small size (one point smaller than the default font)
 bold face.
+This macro is an extension that probably first appeared in SunOS 4.0
+and was later adopted by GNU and by
+.Bx 4.4 .
 .It Ic SH
 Begin a section.
 The scope of a section is only closed by another section or the end of
@@ -435,6 +444,8 @@ Sets the volume for the footer for compa
 .Bx
 releases.
 The optional first argument specifies which release it is from.
+This macro is an extension that first appeared in
+.Bx 3 .
 .It Ic UE
 End a uniform resource identifier block started with
 .Ic UR .
--
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