[backport#15530] doc: Move wallet lock annotations to header

Summary:
faebd2ef40 doc: Move wallet lock annotations to header (MarcoFalke)

Pull request description:

We put the annotations in a central place (the header) as opposed to spreading them over the cpp files, where they easily get outdated.

https://github.com/bitcoin/bitcoin/pull/15530/commits/faebd2ef40bf6c9209110ad4e229e5a207b9c9da

Backport of Core PR15530

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7009