Part of https://github.com/kubernetes/kubernetes/pull/63040 is the
assumption that scheduler cache updates must happen before equivalence
cache updates for any given informer event.

The reason for this is that the equivalence cache implementation checks
the main cache for staleness while holding the equiv. cache write lock.

case 1: If an informer invalidates an equiv. cache entry before the
staleness check, then we know that the main cache update completed.

case 2: If an informer blocks trying to grab the equiv. cache lock, then
invalidation will occur right after the potentially stale update is
written.

This patch adds a note to places where we invalidate the equivalence
cache so that hopefully nobody violates this invariant.