doc: Mention a bug in NetBSD's *gettext functions. * doc/glibc-functions/gettext.texi: Mention buggy handling of empty LANGUAGE environment variable. * doc/glibc-functions/dgettext.texi: Likewise. * doc/glibc-functions/dcgettext.texi: Likewise. * doc/glibc-functions/ngettext.texi: Likewise. * doc/glibc-functions/dngettext.texi: Likewise. * doc/glibc-functions/dcngettext.texi: Likewise.