rpc: Properly document return values (submitblock, gettxout, getblocktemplate, scantxoutset)

Summary: This is a backport of core#20556

Test Plan:

src/bitcoin-cli help submitblock
src/bitcoin-cli help gettxout
src/bitcoin-cli help getblocktemplate
src/bitcoin-cli help scantxoutset

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11337