doc: Cleanup Texinfo macros use in documentation. * macros.texi: Delete @xbackslashchar macro which was needed for Texinfo versions < 5.1 (2013 release date). Alphabetize macros. Document macros more clearly. * func.txi, data.cc, error.cc, file-io.cc, regexp.cc, strfns.cc, utils.cc, pt-eval.cc, beep.m, dlmwrite.m, strread.m, textread.m, asctime.m, ctime.m: Replace "@xbackslashchar" with just "@backslashchar". * graphics.cc (Fwaitfor): Replace '\' in @qcode macro with @backslashchar. * input.cc (FPS1): Replace '\' in @qcode macro with @backslashchar. Use single quotes in @example block to avoid "\\" construct. * jsonencode.cc (Fjsonencode): Replace '\' in @qcode macro with @backslashchar. * sysdep.cc (Fwinqueryreg): Replace '\' in @qcode macro with @backslashchar.

--- a/libinterp/corefcn/strfns.cc	Fri Nov 27 13:49:11 2020 -0500
+++ b/libinterp/corefcn/strfns.cc	Fri Nov 27 17:01:47 2020 -0800
@@ -1078,7 +1078,7 @@
 @deftypefn {} {} newline
 Return the character corresponding to a newline.
 
-This is equivalent to @qcode{"@xbackslashchar{}n"}.
+This is equivalent to @qcode{"@backslashchar{}n"}.
 
 Example Code