ltmain.in: Replace old warnings with documentation

ltmain.in: Replace old warnings with documentation

Without the ability to disable these old warnings individually, they
are migrating to the relevant section of the documentation to inform
new users that may not know. These warnings have been present for
two decades, so existing users should be aware of this functionality.

* build-aux/ltmain.in: Remove old warnings.
* doc/libtool.texi: Document contents of warning messages.