Merge #15321: doc: Add cs_main lock annotations for mapBlockIndex

Summary:
PR15321 backport, aside from one minor change which will be noticed
when the backport in question happens later on ( PR14957 ).

Depends on D5237

Test Plan: ninja check all using clang compiler

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5238