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