Improve documentation of "C-x 8 RET".

Improve documentation of "C-x 8 RET".

 mule.texi (Input Methods): Mention "C-x 8 RET" and add a
 cross-reference to "Inserting Text".
 basic.texi (Inserting Text): Add an index entry for "C-x 8 RET".
 Mention completion provided by `ucs-insert'.