cachefiles: document on-demand read mode

Document new user interface introduced by on-demand read mode.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220509074028.74954-9-jefflexu@linux.alibaba.com


Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>