Merge #13402: Document validationinterace callback blocking deadlock potential.

Summary:
25bc9615b7 Document validationinterace callback blocking deadlock potential. (Matt Corallo)

Pull request description:

From the branches-I've-had-lying-around-and-forgot-to-PR department...

This is a comment-only PR, but the comments point out an API quirk that isn't exactly trivial. None of our use-cases right now hit this, but if we were to call SyncWithValidationInterfaceQueue (eg to limit queue depth) in ATMP, I'm pretty sure we'd hit a deadlock there.

Tree-SHA512: 889dd8fc9eb15d1f2aa5ca467e783bc8f07bc543b166b032741795b0db7a0df11a2846d3cb7c69bafa8d1acf970021001b742f52be06725a932813230c5b4a7b

Backport of Core PR13402
https://github.com/bitcoin/bitcoin/pull/13402/

Depends on D4138

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4139