[backport#16404] doc: Add note regarding ZMQ block notification
[backport#16404] doc: Add note regarding ZMQ block notification

Summary:
https://github.com/bitcoin/bitcoin/pull/16404/commits/6bc1ff915dd495f05985d3402a34dbfc3b6a08b4

Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/16404 | PR16404]]

Test Plan:
  read it

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7342

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