docs: filesystems: caching/backend-api.rst: fix an object withdrawn API

There's no fscache_are_objects_withdrawn() helper at all to test if
cookie withdrawal is completed currently. The cache backend is using
fscache_wait_for_objects() to wait all objects to be withdrawn.

Signed-off-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://listman.redhat.com/archives/linux-cachefs/2022-April/006705.html # v1