Document -Wderiving-typeable
This documents the -Wderiving-typeable warning. This warning has occurred in the Yesod and Persistent codebases which recently fixed it (https://github.com/yesodweb/yesod/issues/1653).
I saw there was a call for fixing some undocumented flags here #18633 so I picked this one up.
I'm having trouble updating my local GHC branch (some timeout error—probably my rural internet), which frustrated testing this locally. I'm hoping I got the syntax all correct though and CI builds the docs for me or something.
Tracking: #18641