doc: Update po_message_t API (VI)
* gettext-tools/doc/gettext.texi (po_message_t API): Add previous
message contents accessors po_message_prev_msgctxt,
po_message_set_prev_msgctxt, po_message_prev_msgid,
po_message_set_prev_msgid, po_message_prev_msgid_plural, and
po_message_set_prev_msgid_plural documentation.
