mandoc: add a BUGS section explaining the situation with box and line

mandoc: add a BUGS section explaining the situation with box and line

[schwarze]

Log Message:
-----------
add a BUGS section explaining the situation with box and line drawing
in UTF-8 output; suggested by bentley@

Modified Files:
--------------
    mandoc:
        tbl.7

Revision Data
-------------
Index: tbl.7
===================================================================
RCS file: /home/cvs/mandoc/mandoc/tbl.7,v
retrieving revision 1.32
retrieving revision 1.33
diff -Ltbl.7 -Ltbl.7 -u -p -r1.32 -r1.33
--- tbl.7
+++ tbl.7
@@ -1,7 +1,7 @@
 .\"	$Id$
 .\"
 .\" Copyright (c) 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
-.\" Copyright (c) 2014, 2015, 2017, 2018 Ingo Schwarze <schwarze@openbsd.org>
+.\" Copyright (c) 2014,2015,2017,2018,2019 Ingo Schwarze <schwarze@openbsd.org>
 .\"
 .\" Permission to use, copy, modify, and distribute this software for any
 .\" purpose with or without fee is hereby granted, provided that the above
@@ -438,3 +438,17 @@ reference was written by
 .An Kristaps Dzonsons Aq Mt kristaps@bsd.lv
 and
 .An Ingo Schwarze Aq Mt schwarze@openbsd.org .
+.Sh BUGS
+In
+.Fl T
+.Cm utf8
+output mode, heavy lines are drawn instead of double lines.
+This cannot be improved because the Unicode standard only provides
+an incomplete set of box drawing characters with double lines,
+whereas it provides a full set of box drawing characters
+with heavy lines.
+It is unlikely this can be improved in the future because the box
+drawing characters are already marked in Unicode as characters
+intended only for backward compatibility with legacy systems,
+and their use is not encouraged.
+So it seems unlikely that the missing ones might get added in the future.
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