doc: man: consistently format -X[OPTIONAL] form

This is significant for the date, od, and pr commands
which have options of the form -X[OPTIONAL], which change like:

  diff -r man.orig/date.1 man/date.1
  < \fB\-I[FMT]\fR, \fB\-\-iso\-8601\fR[=\fI\,FMT\/\fR]
  > \fB\-I\fR[\fI\,FMT\/\fR], \fB\-\-iso\-8601\fR[=\fI\,FMT\/\fR]

  diff -r man.orig/od.1 man/od.1
  < \fB\-w[BYTES]\fR, \fB\-\-width\fR[=\fI\,BYTES\/\fR]
  > \fB\-w\fR[\fI\,BYTES\/\fR], \fB\-\-width\fR[=\fI\,BYTES\/\fR]

* man/help2man (convert_options): Support options of the form
-X[PARAM], so that we now consistently format them (in italics).

man/help2man

@@ -803,7 +803,9 @@ sub convert_option
     local $_ = '\fB' . shift;
 
     s/-/\x83/g;
-    unless (s/\[=(.*)\]$/\\fR[=\\fI$1\\fR]/)
+    unless (
+        s/\x83(\w)\[(.*)\]/\x83$1\\fR[\\fI$2\\fR]/ or  # short form [optional]
+        s/\[=(.*)\]$/\\fR[=\\fI$1\\fR]/)               # long form [optional]
     {
 	s/=(.)/\\fR=\\fI$1/;
 	s/ (.)/ \\fI$1/;