docs: filesystems: caching/backend-api.rst: fix an object withdrawn API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fscache_wait_for_objects() to wait all objects to be withdrawn.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

Author: taigerhu

Documentation/filesystems/caching/backend-api.rst

@@ -110,9 +110,9 @@ to withdraw them, calling::
 
 on the cookie that each object belongs to.  This schedules the specified cookie
 for withdrawal.  This gets offloaded to a workqueue.  The cache backend can
-test for completion by calling::
+wait for completion by calling::
 
-	bool fscache_are_objects_withdrawn(struct fscache_cookie *cache);
+	void fscache_wait_for_objects(struct fscache_cache *cache);
 
 Once all the cookies are withdrawn, a cache backend can withdraw all the
 volumes, calling::