RPC: Fix more formatting nits

Summary:

listsinceblock had a double ellipsis (elision + continuation); this looks ugly, just one is needed.
Elision ellipsis wasn't getting a comma, so was truncated to ".." by comma-removal code.
Elision ellipsis was indented incorrectly (as if it was a subitem).
Similarly, type "none" would get truncated to "Non", when it should really be "null" anyway.

This is a backport of Core PR18508

Related to D8892

Test Plan:
ninja all check-all

Check the help message for a list of commands from https://github.com/bitcoin/bitcoin/pull/18508#issuecomment-608610053

src/bitcoin-cli help mockscheduler    # None -> null
src/bitcoin-cli help getblock         # ... -> ...,
src/bitcoin-cli help decodepsbt       # .. -> ...

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8900