Clarify intended usage of the license file modules. Reported by Asher Gordon <AsDaGo@posteo.net> in <https://lists.gnu.org/archive/html/bug-gnulib/2020-05/msg00126.html>. * doc/licenses-texi.texi (License Texinfo sources): Mention the GNU AGPL. Explain the intended usage of the modules. * modules/fdl (Notice): Discourage use as a module. * modules/fdl-1.3 (Notice): Likewise.