This is a backport of core#19521 [5/17]
https://github.com/bitcoin/bitcoin/pull/19521/commits/3c914d58ff323255b32e717d0ce28209ec0abdaa
partial https://github.com/bitcoin/bitcoin/pull/19521/commits/6a4c0c09ab4d073a26c3c4a02783d5dcd88f6eef
The functional test only checks that "-coinstatsindex" argument does not break anything. The rest of the functional test from commit 6a4c0c09ab is not yet applicable and will be added in the next commit.
This includes also minor documentation fixups from core#21818