index: Coinstats index can be activated with command line flag

Summary:
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

Depends on D11598 and D11595

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11599