Document the use of macros in %error and %warning, valid since 2.03.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
\c %endif
It is optional for the message string after \c{%error} or \c{%warning}
-to be quoted.
+to be quoted. If it is \e{not}, then single-line macros are expanded
+in it, which can be used to display more information to the user. For
+example:
+
+\c %if foo > 64
+\c %assign foo_over foo-64
+\c %error foo is foo_over bytes too large
+\c %endif
\H{otherpreproc} \i{Other Preprocessor Directives}