[backport#15508] Add documentation of struct PSBTAnalysis et al

Summary:
https://github.com/bitcoin/bitcoin/pull/15508/commits/892eff05f115c0b002d0e0b6ffc3ab418480d25c

Depends on D6079

This completes the backport of Core PR15508

Test Plan: none

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6080