doc: Patterns in m4_pattern_forbid cause error, not warning
The example text regarding a desired literal AC_DC in output
claimed that the result would trigger a warning if one does
not use creative quoting; but in reality, autoconf's use of
m4_pattern_forbid to reserve the entire AC_ namespace makes
it a hard error. Reword the section to mention the use of
m4_pattern_allow() as the fix, and beef up the example to
better demonstrate the problem.
* doc/autoconf.texi (Autoconf Language): Improve AC_DC example.
Reported by Gavin Smith <gavinsmith0123@gmail.com>.
Signed-off-by: Eric Blake <eblake@redhat.com>