[avalanche] Add comment to clarify pm::exists

Summary:
The addition of danglingProofPool made the API of this function less clear.
Renaming the function is debatable so I prefer to just comment it and move on.

Test Plan: Read the comment and verify the implementation matches those expectations.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16602