[backport#17154 1/3][wallet] Add doxygen comment to CWallet::CommitTransaction()

Summary:
https://github.com/bitcoin/bitcoin/pull/17154/commits/b6f486a02b463ffeaf82ec11fc6f74f439c037ae

note: the first commit
(https://github.com/bitcoin/bitcoin/pull/17154/commits/8bba91b22d22a8dfea7c947b542b1022bfc1c0ea) didn't really apply to us.

Depends on D7111

Partial backport of Core PR17154

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7112