Clarify comment for SCRIPT_ENABLE_CHECKDATASIG

Summary:
The important function of the flag, now that it is activated,
is to flag when it should be counted towards the sigops limit.

Test Plan: make check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D3060