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