Better documentation for TSAN suppressions

[ngoldbaum]

CPython has a suppressions file, so does NumPy, people will probably end up writing their own suppressions file and may need to combine suppressions specific to a project with suppressions for known issues that are already documented in upstream suppression files.

First, we should document the current situation and link to the locations of upstream suppression files.

Second, we should add narrative guidance about how to handle and think about suppressions. We should also add guidance about what to do if you suspect you've found a real race in CPython or a library that ships extensions.

Last, it might be neat to think about a webapp or something like that to generate suppressions files by concatenating the existing suppressions shipped by projects. It might also be neat to scan github to look for out-of-date suppressions.