doc: Document how to use custom *.its files.

doc: Document how to use custom *.its files.

Reported by Stefan Brüns <stefan.bruens@rwth-aachen.de>
at <https://savannah.gnu.org/bugs/?64026>.

* gettext-tools/src/search-path.c (get_search_path): Improve comments.
* gettext-tools/doc/xgettext.texi: New subsection "Options for XML input files".
Move the description of --its out of subsection "Output details".
* gettext-tools/doc/msgfmt.texi: Document GETTEXTDATADIRS.
* gettext-tools/doc/gettext.texi (Installers): Document the
PREFIX/share/gettext/its/ directory.