mqtt: send ping at upkeep interval

Closes #16975

docs/internals/MQTT.md

@@ -26,6 +26,10 @@ Example subscribe:
 This sends an MQTT SUBSCRIBE packet for the topic `bedroom/temp` and listen in
 for incoming PUBLISH packets.
 
+You can set the upkeep interval ms option to make curl send MQTT ping requests to the
+server at an internal, to prevent the connection to get closed because of idleness.
+You might then need to use the progress callback to cancel the operation.
+
 ### Publishing
 
 Command usage:

docs/libcurl/curl_easy_upkeep.md

@@ -31,10 +31,13 @@ send some traffic on existing connections in order to keep them alive; this
 can prevent connections from being closed due to overzealous firewalls, for
 example.
 
-Currently the only protocol with a connection upkeep mechanism is HTTP/2: when
+For HTTP/2 we have an upkeep mechanism: when
 the connection upkeep interval is exceeded and curl_easy_upkeep(3)
 is called, an HTTP/2 PING frame is sent on the connection.
 
+For MQTT the upkeep interval defines when to send ping requests to prevent the
+server from disconnecting.
+
 This function must be explicitly called in order to perform the upkeep work.
 The connection upkeep interval is set with
 CURLOPT_UPKEEP_INTERVAL_MS(3).