Fix docs on multiplayer peer signals.

NetworkedMultiplayerPeerENet emits peer_connected and peer_disconnected
on both the client and the server:
https://github.com/godotengine/godot/blob/6fed1ffa313c6760fa88b368ae580378daaef0f0/modules/enet/networked_multiplayer_enet.cpp#L268

When trying to implement `NetworkedMultiplayerCustom`, I followed the
documentation and only emitted this signal on the server.
I ended up getting errors like:

```
Invalid packet received. Unabled to find requested cached node
```

While I didn't check other peer implementations, it seems that emitting
on both the client and server is required.

I copied the wording from the `master` branch documentation.

Here's some output from a test program with all the signals connected:
```
1948301815 got multiplayer.network_peer_connected from  1
1948301815 got peer.peer_connected from 1
1948301815 got multiplayer.connected_to_server
1948301815 got peer.connection_succeeded
1413532890 got multiplayer.network_peer_connected from  1
1413532890 got peer.peer_connected from 1
1413532890 got multiplayer.connected_to_server
1413532890 got peer.connection_succeeded
1 got multiplayer.network_peer_connected from  1413532890
1 got peer.peer_connected from 1413532890
1 got multiplayer.network_peer_connected from  1948301815
1 got peer.peer_connected from 1948301815
1413532890 got multiplayer.network_peer_connected from  1948301815
1413532890 got peer.peer_connected from 1948301815
1948301815 got multiplayer.network_peer_connected from  1413532890
1948301815 got peer.peer_connected from 1413532890
1 got multiplayer.network_peer_disconnected from  1948301815
1 got peer.peer_disconnected from 1948301815
1413532890 got multiplayer.network_peer_disconnected from  1948301815
1413532890 got peer.peer_disconnected from 1948301815
1 got multiplayer.network_peer_disconnected from  1413532890
1 got peer.peer_disconnected from 1413532890
```

doc/classes/NetworkedMultiplayerPeer.xml

@@ -67,13 +67,13 @@
 		<signal name="peer_connected">
 			<argument index="0" name="id" type="int" />
 			<description>
-				Emitted by the server when a client connects.
+				Emitted when a remote peer connects.
 			</description>
 		</signal>
 		<signal name="peer_disconnected">
 			<argument index="0" name="id" type="int" />
 			<description>
-				Emitted by the server when a client disconnects.
+				Emitted when a remote peer has disconnected.
 			</description>
 		</signal>
 		<signal name="server_disconnected">