Add a gotcha about FactoryGirl sequences
What does this MR do?
We had a weird spec failure tracked by https://gitlab.com/gitlab-org/gitlab-ce/issues/24341 for which FactoryGirl sequence feature made the problem even harder to understand.
This MR aims to highlight this gotcha when using sequence-generated attributes. We might even want to get rid of sequences in the future to avoid such potential issues...