doc: Don't expose generated version
* gettext-tools/doc/Makefile.am ($(srcdir)/stamp-vti): Set
ARCHIVE-VERSION, derived from $(ARCHIVE_VERSION) in version.texi.
* gettext-tools/doc/gettext.texi (Files under Version Control): Refer to
ARCHIVE-VERSION, instead of VERSION.
* gettext-tools/doc/autopoint.texi: Refer to ARCHIVE-VERSION, instead of VERSION.
