[doc] Provide rationale for randomization in scheduling.

Summary:
This is a backport of Core PR18807 [5/9]
https://github.com/bitcoin/bitcoin/pull/18807/commits/1f94bb0c744a103b633c1051e8fbc01e612097dc

Test Plan: ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9032