Refine the 'define' documentation

A while ago, an AdaCore user reported some difficulties with the
'define' command.  While some of these difficulties are intrinsic, or
at least difficult to change, it seemed sensible to document a couple
of the typical problems -- and to make the text describing argument
substitution a bit more prominent.

Approved-By: Eli Zaretskii <eliz@gnu.org>