Document how to identify feature flag-related test failures
We could improve our ability to identify feature flag changes so that we can know if a test failure might have been caused by one.
Some details in gitlab-org/gitlab#273007 (comment 448555162)
For example, I only just learned about the feature-flag-log, which could be very handy to correlate the time a test started failing on Staging or Prod with the time a feature flag was changed.
There's likely other information I'm not aware of yet that could be useful.
I think we should add a section to the relevant part of the handbook on how to determine if a failure could be related to a feature flag change.