mandoc: Explain the ASCII rendering of single quotes because that

mandoc: Explain the ASCII rendering of single quotes because that

[schwarze]

Log Message:
-----------
Explain the ASCII rendering of single quotes because that repeatedly
caused confusion in the past.  People plainly do not expect that 
there are limits to the compatibility between Unicode and ASCII,
but there are.

The information belongs here and not into mandoc_char(7) because
it explains how the specific output device (-T ascii) works and
because it has nothing to do with the question of how characters
are represented on the input side.

Modified Files:
--------------
    mandoc:
        mandoc.1

Revision Data
-------------
Index: mandoc.1
===================================================================
RCS file: /home/cvs/mandoc/mandoc/mandoc.1,v
retrieving revision 1.236
retrieving revision 1.237
diff -Lmandoc.1 -Lmandoc.1 -u -p -r1.236 -r1.237
--- mandoc.1
+++ mandoc.1
@@ -267,6 +267,17 @@ instead.
 The special characters documented in
 .Xr mandoc_char 7
 are rendered best-effort in an ASCII equivalent.
+In particular, opening and closing
+.Sq single quotes
+are represented as characters number 0x60 and 0x27, respectively,
+which agrees with all ASCII standards from 1965 to the latest
+revision (2012) and which matches the traditional way in which
+.Xr roff 7
+formatters represent single quotes in ASCII output.
+This correct ASCII rendering may look strange with modern
+Unicode-compatible fonts because contrary to ASCII, Unicode uses
+the code point U+0060 for the grave accent only, never for an opening
+quote.
 .Pp
 The following
 .Fl O
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