doc: Clarify locale names and language names.
Reported by Arfrever <arfrever.fta@gmail.com>
at <https://savannah.gnu.org/bugs/?59062>.
* gettext-runtime/doc/nls.texi (INSTALL Matters, Using This Package): Mention
ll_CC as alterntive to ll, and that ll can be a 3-letter code.
* gettext-tools/doc/gettext.texi (Locale Names, Header Entry, Installers):
Likewise.
