[backport#15122] [RPC] Expand help text for importmulti changes

Summary:
[docs] Expand help text for importmulti changes (John Newbery)

Pull request description:

Expands the RPC help text for changes to the importmulti RPC method.

https://github.com/bitcoin/bitcoin/pull/15122/commits/b745e149c26507e31cdaed0412dcd9963647568d

Backport of Core PR15122

Test Plan:

ninja

read the RPC help

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7325