doc: document the use of macros in %error

Document the use of macros in %error and %warning, valid since 2.03.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/doc/nasmdoc.src b/doc/nasmdoc.src
index 964b6f4f..ffd416a4 100644
--- a/doc/nasmdoc.src
+++ b/doc/nasmdoc.src
@@ -3421,7 +3421,14 @@ Similarly, \c{%warning} issues a warning, but allows assembly to continue:
 \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}