esp32/modnetwork: Add max_clients kw-arg to WLAN.config for AP setting.

This allows the user to configure the maximum number of clients that are
connected to the access point.  Resolves #5125.

docs/esp32/quickref.rst

@@ -82,6 +82,7 @@ The :mod:`network` module::
 
     ap = network.WLAN(network.AP_IF) # create access-point interface
     ap.config(essid='ESP-AP') # set the ESSID of the access point
+    ap.config(max_clients=10) # set how many clients can connect to the network
     ap.active(True)         # activate the interface
 
 A useful function for connecting to your local WiFi network is::