RPC: Fix more formatting nits
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 [[https://github.com/bitcoin/bitcoin/pull/18508 | 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

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