document MANDATORY_SCRIPT_VERIFY_FLAGS accurately

Summary:
Currently MANDATORY_SCRIPT_VERIFY_FLAGS is used in weird ways and also
its documentation is a misleading lie. I'd like to fix the craziness,
which starts with documenting it accurately.

Test Plan: read

Reviewers: Mengerian, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4672