Suggested wallet code cleanups from #14711
Summary:
This implements suggested changes from #14711 review comments that didn't make make it in before merging. There are no changes in behavior in this PR, just documentation updates, simplifications, and variable renames.
Backport of core PR15342.
Test Plan:
ninja all check check-functional
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D5953