Clarify which point in each branch is compared in a pull request

[megbird]

Some users are confused about the difference in the commits displayed shown on the compare page and what's shown in a PR.

We talk about what a compare page shows in "About comparing branches in pull requests;" we can add some simple to "About pull requests."

Compare pages show a compare between the tip of the head ref and the current common ancestor (i.e. merge base) of the head and base ref. On the other hand, pull request pages show a compare between the tip of the head ref and the common ancestor of the head and base ref at the time when the pull request was created. In other words, the merge base used for the comparison might be different.

[Ruth-ikegah]

I want to take this up!