doc: Recommend to avoid literal URLs in translatable strings.
Suggested by John Darrington <john@darrington.wattle.id.au>
in <https://lists.gnu.org/archive/html/bug-gnulib/2019-05/msg00033.html>.
* gettext-tools/doc/gettext.texi (Preparing Strings): Add recommendation against
embedded URLs. Add sub-headers.
