docs: filesystems: caching/backend-api.rst: correct two relinquish APIs use

1. cache backend is using fscache_relinquish_cache() rather than
   fscache_relinquish_cookie() to reset the cache cookie.

2. No fscache_cache_relinquish() helper currently, it should be
   fscache_relinquish_cache().

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/006703.html # v1
Link: https://listman.redhat.com/archives/linux-cachefs/2022-April/006704.html # v2