netfs: Update main API document

Bring the netfs documentation up to date.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/1690127.1744208325@warthog.procyon.org.uk


Reviewed-by: "Paulo Alcantara (Red Hat)" <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Viacheslav Dubeyko <slava@dubeyko.com>
cc: Alex Markuze <amarkuze@redhat.com>
cc: Timothy Day <timday@amazon.com>
cc: Jonathan Corbet <corbet@lwn.net>
cc: netfs@lists.linux.dev
cc: linux-doc@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>