When getting a warning that we release a capability that isn't held it's
sometimes not clear why. So just like we do for double locking, we add a
note on the previous release operation, which marks the point since when
the capability isn't held any longer.
We can find this previous release operation by looking up the
corresponding negative capability.
The parameter itself isn't optional, so the "optionally" seems a bit strange to me. I think it should say, If the source location is valid, it represents the location of a previous Unlock, or rework the interface to use an llvm::Optional.