Update Future Development section in manual.

* doc/todo.texi (Mid-term Goals): Remove "More flexible option and
command handling" item: we now have `TeX-command-extra-options'.
(Wishlist): Remove "Poor man's Source Specials": AUCTeX supports
source specials and SyncTeX.
(Wishlist): Remove "multiple completion for \bibliography" item:
"\bibliography" does complete multiple arguments.