doc: Add copyright notices in several files.
Reported by <ineiev@gnu.org> in <https://savannah.gnu.org/bugs/?54809>.
* gettext-runtime/doc/nls.texi: Add delegating copyright notice.
* gettext-runtime/doc/rt-*.texi: Likewise.
* gettext-tools/doc/autopoint.texi: Likewise.
* gettext-tools/doc/gettextize.texi: Likewise.
* gettext-tools/doc/msg*.texi: Likewise.
* gettext-tools/doc/xgettext.texi: Likewise.
