todo-mode.el: Remove dependence on auto-mode-alist.

todo-mode.el: Remove dependence on auto-mode-alist.

* calendar/todo-mode.el: Remove dependence on auto-mode-alist,
to avoid errors when trying to create or visit a file foo.todo
located outside to todo-directory, and to allow having such files
without them being tied to Todo mode.
(todo-show, todo-move-category, todo-merge-category, todo-find-archive)
(todo-archive-done-item, todo-find-filtered-items-file)
(todo-filter-items, todo-find-item, todo-diary-goto-entry)
(todo-category-completions, todo-read-category): When visiting a
Todo file, make sure we're in the right mode and the buffer local
variables are set.
(todo-make-categories-list, todo-reset-nondiary-marker)
(todo-reset-done-string, todo-reset-comment-string): After
processing all Todo files, kill the buffers of those files that
weren't being visited before the processing.
(todo-display-as-todo-file, todo-add-to-buffer-list)
(todo-visit-files-commands): Comment out.
(todo-modes-set-3, todo-mode): Comment out additions to find-file-hook.
(auto-mode-alist): Remove add-to-list calls making Todo file
extensions unrestrictedly tied to Todo modes.

* doc/misc/todo-mode.texi: Update in light of changes due to bug#17482.
Replace numerous mistaken uses of literal quotes with proper
Texinfo markup.
(Todo Mode Entry Points): Comment out reference to using find-file
or Dired to visit Todo files, since this has been disabled (bug#17482).