automake: recommend autoreconf -f with version mismatches.

automake: recommend autoreconf -f with version mismatches.

From https://bugs.gnu.org/68674.

* bin/automake.in (scan_autoconf_traces): change error
to suggest running autoreconf -f. The version mismatch is
most likely to happen due to the cache files, which
aclocal && automake (the previous recommendation) does not delete.
(scan_autoconf_files): likewise.
* doc/automake.texi (Auto-generating aclocal.m4): mention autoreconf.
Other minor wording changes.