Add a documentation about Awa_mirage.spawn_server and its usage

mirage/awa_mirage.mli

@@ -42,6 +42,26 @@ module Make (F : Mirage_flow.S) (T : Mirage_time.S) (M : Mirage_clock.MCLOCK) :
 
   val spawn_server : ?stop:Lwt_switch.t -> Awa.Server.t -> Awa.Ssh.message list -> F.flow ->
     exec_callback -> t Lwt.t
+  (** [spawn_server ?stop server msgs flow callback] launches an {i internal}
+      SSH channels handler which can be stopped by [stop]. This SSH channels
+      handler will call [callback] for every new channels requested by the
+      client. [msgs] are the SSH {i hello} given by {!val:Awa.Server.make} which
+      returns also a {!type:Awa.Server.t} required here.
+
+      A basic usage of [spawn_server] is:
+      {[
+        let ssh_channel_handler _cmd _ic _oc _ec =
+          Lwt.return_unit
+
+        let tcp_handler flow =
+          let server, msgs = Awa.Server.make private_key db in
+          SSH.spawn_server server msgs flow ssh_handler >>= fun _t ->
+          close flow
+      ]}
+
+      {b NOTE}: Even if the [ssh_channel_handler] is fulfilled, [spawn_server]
+      continues to handle SSH channels. Only [stop] can really stop the internal
+      SSH channels handler. *)
 
 end
   with module FLOW = F