Type documentation fixes for Doc

[svlandeg]

Description

• Fix doc.pyi to include the fact that char_span() can return None
• Unify type documentation of exclude: it's a tuple by default, originally documented to be a List. Can be documented as an Iterable[str], like we do for util.to_dict and the to_disk method in doc.pyi.

Types of change

docs fixes

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.