[backport#16866] wallet: Rename 'decode' argument in gettransaction method to 'verbose'

Summary:
[wallet] Rename 'decode' argument in gettransaction method to 'verbose' (John Newbery)

Pull request description:

This makes the RPC method consistent with other RPC methods that have a
'verbose' option.

Change the name of the return object from 'decoded' to details.

Update help text.

Backport of Core PR16866 and PR16873 (required to unbreak PR16866)

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7378