netfs: Documentation for helper library

Add interface documentation for the netfs helper library.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-mm@kvack.org
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/161539533275.286939.6246011228676840978.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653791767.2770958.2012814194145060913.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789072591.6155.9448294406920216219.stgit@warthog.procyon.org.uk/ # v6

Documentation/filesystems/index.rst

@@ -53,6 +53,7 @@ filesystem implementations.
    journalling
    fscrypt
    fsverity
+   netfs_library
 
 Filesystems
 ===========