Merge #16873: rpc: fix regression in gettransaction

1b41c2c8a126ef4be183e1d800a17d85cab8837b test: improve gettransaction test coverage (Jon Atack)
0f34f54888f680bfbe7a29ac278636d7178a99bb rpc: fix regression in gettransaction (Jon Atack)

Pull request description:

  Closes #16872.

  PR #16866 renamed the `decode` argument in gettransaction to `verbose` to make it more consistent with other RPC calls like getrawtransaction. However, it inadvertently overloaded the "details" field when `verbose` is passed. The result is that the original "details" field is no longer returned correctly, which seems to be a breaking API change.

  This PR:

  - takes the simplest path to restoring the "details" field by renaming the decoded one back to "decoded" while leaving the `verbose` argument for API consistency, which was the main intent of #16866,

  - addresses [this comment](https://github.com/bitcoin/bitcoin/pull/16185#discussion_r320740413) by mentioning in the RPC help that the new decoded field is equivalent to decoderawtransaction, and

  - updates the help, functional test, and release note.

  Reviewers, to test this manually, build and run `bitcoin-cli help gettransaction` and `bitcoin-cli gettransaction <wallet txid> false true`, and verify that the command returns both `details` and `decoded` fields.

ACKs for top commit:
  jnewbery:
    tACK 1b41c2c8a126ef4be183e1d800a17d85cab8837b

Tree-SHA512: 287edd5db7ed58fe8b548975aba58628bd45ed708b28f40174f10a35a455d89f796fbf27430aa881fc376f47aabda8803f74d4d100683bd86577a02279091cf3

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