mount: clarify that umount retries do not (anymore) allow multiple timeouts

mount: clarify that umount retries do not (anymore) allow multiple timeouts

It _looks_ as if, back when we used to retry unsuccessful calls to umount,
this would have inflated the effective timeout.  Multiplying it by
RETRY_UMOUNT_MAX.  Which is set to 32.

I'm surprised if it's true: I would have expected it to be noticed during
the work on NFS timeouts.  But I can't see what would have stopped it.

Clarify that I do not expect this to happen anymore.  I think each
individual umount call is allowed up to the full timeout, but if umount
ever exited with a signal status, we would stop retrying.

To be extra clear, make sure that we do not retry in the event that umount
perversely returned EXIT_SUCCESS after receiving SIGTERM.