Merge #11083: Fix combinerawtransaction RPC help result section

Summary:
PR11083 backport https://github.com/bitcoin/bitcoin/pull/11083/files
f9ca0fe Fix combinerawtransaction RPC help result section (Jonas Nick)

Pull request description:

Without this PR it looks like the RPC would return something like a dictionary. But it just returns the transaction in hex.

Test Plan:
make check
read

Reviewers: deadalnix, jasonbcox, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3481