Merge #9333: Document CWalletTx::mapValue entries and remove erase of nonexistent "version" entry.

Summary:
backport PR9333 https://github.com/bitcoin/bitcoin/pull/9333/files
87ed396 [trivial] Add comment documenting bumpfee mapValues (Russell Yanofsky)
a1fe944 Remove reference to nonexistent "version" wallet transaction mapvalue field (Russell Yanofsky)
654e044 [trivial] Add comment documenting CWalletTx::mapValue (Russell Yanofsky)

Tree-SHA512: 1fd1860e345c59b13634db2007fff4ba30aaf1f177fdd765f47bf9257fac117cdcd5d491424416da304c08e85effbb27f3424f072f7c9587ef39cb98531b932a

Test Plan:
make check
test-runner.py

• note, as mentioned by a1fe944 commit message, the line about "version" was introduced by accident in olden times (865c3a2) and the "version" field never actually got used (and it's certainly mentioned nowhere else in codebase)

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2984