index, rpc: Coinstatsindex follow-ups

Summary:

Index: Return early from failed coinstatsindex init

https://github.com/bitcoin/bitcoin/pull/22047/commits/01386bfd88019397237256cb16f91de346eb66f2

rpc: Add missing gettxoutsetinfo help docs

https://github.com/bitcoin/bitcoin/pull/22047/commits/a5f6791139554936d13f367660283899a37ff5c7

rpc: Block until synced if coinstatsindex is used in gettxoutsetinfo

During initial sync after startup the gettxoutsetinfo RPC will still return an error while catching up. However, after the initial sync the index will not error immediately anymore when it's in the process of syncing to the tip while being called. Instead it will block until synced and then return the response.

https://github.com/bitcoin/bitcoin/pull/22047/commits/d4356d4e48f59c63894b68691cc21ed4892ee716

Index: Improve logging in coinstatsindex

More accurate logging of a warning should make clear if the recovery condition was hit while catching the results of the previous block.

https://github.com/bitcoin/bitcoin/pull/22047/commits/5b3d4e724f377834e24b1f014787cc7aa7fc30fe

coinstats: Add comments for new coinstatsindex values

https://github.com/bitcoin/bitcoin/pull/22047/commits/779e638ca9b2b37c247577d225b93ac762b0602f

This concludes backport of core#22047

Some commits were already backported as dependencies of other backports in D11597 and D11605

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13906