fscache: Update the documentation to reflect I/O API changes

Update the fscache documentation to remove the old I/O API bits and to note
the new fallback API.

Changes
=======
ver #2:
  - Changed "deprecated" to "fallback" in the new function names[1].

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/CAHk-=wiVK+1CyEjW8u71zVPK8msea=qPpznX35gnX+s8sXnJTg@mail.gmail.com/ [1]
Link: https://lore.kernel.org/r/163162778200.438332.1918683687532006409.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/163189115518.2509237.6454712882112339524.stgit@warthog.procyon.org.uk/ # rfc v2