Summary:
Non-functional comment-only changes.

Goal: Adjust Promise's code-docs so they are consistent with Future and SemiFuture, e.g., change "valid() must be true" to "`valid() == true`"

Reviewed By: LeeHowes

Differential Revision: D9415137

fbshipit-source-id: dbb8df00919779b339e5b08337eba0df1a210ac2