Document that caches should be cleared after calling field_attach_update()

There have been a number of cases where contrib modules call field_attach_update() to update field values without saving an entity. This leads to stale caches unless the contrib module manually resets the cache.

The documentation for field_attach_update() should mention that caches need to be cleared.

See this issue in the Entity Cache queue for details:
#1966164: Consider implementing hook_field_attach_update() to clear cache when field values are updated