This is a bit specific, and I'm not clear it is that helpful.
IMO the point here is simple to explain that a PR is just the start of a dialogue : We don't necessarily need all possible relevant info up-front, because we can ask for clarification if needed. 
In fact what I'd like here is to project a comforting sense that submissions don't need to be **perfect**.
Perhaps...
"""
The point of the checklist is to explain what aspects must be covered before a PR can be merged.  You don't have to consider every possible angle in your initial submission, but proper consideration of these points obviously saves time and will also help encourage a developer to take up it in the first place.
"""