Warn about PR problems in docs. Refs #16, #36, #350.

[pkienzle]

I assert that this patch is my own work, and to simplify the licensing of the BFG Repo-Cleaner:

(choose 1 of these 2 options)

• I assign the copyright on this contribution to Roberto Tyley
• I disclaim copyright and thus place this contribution in the public domain

I ran BFG on a repo with pull requests, which was ultimately not useful since the history I was trying to remove is still accessible. This is already reported in #16, #36, #350. I want to warn future me (and other users) that this was the case so I updated the docs.

I suggest rebasing all dependent PRs on the revised commit hashes. I have not yet tried it but it ought to help.

[pkienzle]

Okay, I did try updating the head of the PR to the new commit hash (the easy way, with a force push to the corresponding commit number) but the details of the already merged PR did not change. I removed my suggested mitigation, but left the references to the issues where the problem is discussed.