Merge #10655: Properly document target_confirmations in listsinceblock

Summary:
PR10655 backport https://github.com/bitcoin/bitcoin/pull/10655/files
9f8a46f Properly document target_confirmations in listsinceblock (Ryan Havar)

Pull request description:

There seems to be some misunderstandings about this, but it's a heavily used function so I'd like to make sure the docs are clear about how it works.

For a later issue:
* Change the default of target_confirmations to 6  (1 is a pretty silly default)
* Change the name of target_confirmations (it's really a horrible name)

Test Plan:
make check
read

Reviewers: deadalnix, jasonbcox, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3478