watch_queue: Drop references to /dev/watch_queue

The merged API doesn't use a watch_queue device, but instead relies on
pipes, so let the documentation reflect that.

Fixes: f7e47677 ("watch_queue: Add a key/keyring notification facility")
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Ben Boeckel <mathstuf@gmail.com>