[doc] explain why CheckBlock() is called before AcceptBlock()

Summary:
Co-authored-by: Suhas Daftuar <sdaftuar@gmail.com>

This is a backport of core#15545

Test Plan: proof-reading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12694