[wallet] Rename 'decode' argument in gettransaction method to 'verbose'

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.

src/rpc/client.cpp

@@ -85,7 +85,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "getblockheader", 1, "verbose" },
     { "getchaintxstats", 0, "nblocks" },
     { "gettransaction", 1, "include_watchonly" },
-    { "gettransaction", 2, "decode" },
+    { "gettransaction", 2, "verbose" },
     { "getrawtransaction", 1, "verbose" },
     { "createrawtransaction", 0, "inputs" },
     { "createrawtransaction", 1, "outputs" },