Add more info on PGM's documentation as we ran into some confusion on the activation and probabilistic checks.
Committed 267433@main (7aea8b03c8ef): <https://commits.webkit.org/267433@main> Reviewed commits have been landed. Closing PR #17205 and removing active labels.
<rdar://problem/114668456>
PR: https://github.com/WebKit/WebKit/pull/17205