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.

1 files changed, 13 insertions, 2 deletions

diff --git a/usr.bin/mandoc/mandoc.1 b/usr.bin/mandoc/mandoc.1
index df674370fde..87d26ee1c87 100644
--- a/usr.bin/mandoc/mandoc.1
+++ b/usr.bin/mandoc/mandoc.1
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: mandoc.1,v 1.160 2019/01/01 08:18:06 schwarze Exp $
+.\"	$OpenBSD: mandoc.1,v 1.161 2019/02/23 18:52:45 schwarze Exp $
 .\"
 .\" Copyright (c) 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
 .\" Copyright (c) 2012, 2014-2018 Ingo Schwarze <schwarze@openbsd.org>
@@ -15,7 +15,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: January 1 2019 $
+.Dd $Mdocdate: February 23 2019 $
 .Dt MANDOC 1
 .Os
 .Sh NAME
@@ -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