Remove ", default X" from docstrings

[larsoner]

these should be removed from the docstring sources, IMO. AFAIR our convention is to state the default in the param description prose, not in the type description. Can be a TODO for a follow-up.

Originally posted by @drammock in #14056 (comment)

[larsoner]

And

"optional" is also not something we're supposed to do according to the contrib guide (item 2)