Fix code and doc involving marked items.

Fix code and doc involving marked items.

* todo-mode.texi (Marked Items): Correct omission of item deletion
from commands applying to both todo and done items.

* calendar/todo-mode.el: Fix handling of marked items and make
minor code improvements.
(todo-edit-item): If there are marked items, ensure user can only
invoke editing commands that work with marked items.
(todo-edit-item--text): When there are marked items, make it a
noop if invoked with point not on an item; otherwise, ensure it
applies only to item at point.
(todo-item-undone): If there are marked not-done items, return
point to its original position before signaling user error.
(todo--user-error-if-marked-done-item): New function.
(todo-edit-item--header, todo-edit-item--diary-inclusion)
(todo-item-done): Use it.