Make with_doc_mut take a mutable reference to doc handle

[gterzian]

The with_doc_mut method of DocHandle requires having an immutable reference to the doc handle on which it is called. Internally, it takes a write lock on the document.

This makes it possible to get a deadlock by calling with_doc_mut within with_doc_mut. If we change the method to take a mutable reference such code will not compile.