doc: Update po_message_t API (III)
* gettext-tools/doc/gettext.texi (po_message_t API): Add
po_message_set_msgctxt, po_message_set_msgid,
po_message_set_msgid_plural, po_message_set_msgstr,
po_message_set_msgstr_plural, po_message_set_comments, and
po_message_set_extracted_comments documentation.
