libtextstyle: Document the hyperlink support.

libtextstyle: Document the hyperlink support.

* libtextstyle/doc/libtextstyle.texi (The NO_TERM_HYPERLINKS variable): New
section.
(Hyperlinks): New subsection.
(The abstract styled_ostream class): Document styled_ostream_get_hyperlink_ref,
styled_ostream_get_hyperlink_id, styled_ostream_set_hyperlink.
(The term_ostream class): Document term_ostream_get_hyperlink_ref,
term_ostream_get_hyperlink_id, term_ostream_set_hyperlink.
(The html_ostream class): Document html_ostream_get_hyperlink_ref,
html_ostream_set_hyperlink_ref.
* libtextstyle/NEWS: Mention it.