Move mg_mqtt_next-* to the example, and some nits

docs/README.md

@@ -1799,81 +1799,6 @@ Return value: None
 mg_mqtt_sub(c, mg_str("my/topic"), 1);
 ```
 
-### mg\_mqtt\_next\_sub()
-
-```c
-size_t mg_mqtt_next_sub(struct mg_mqtt_message *msg, struct mg_str *topic, uint8_t *qos, size_t pos);
-```
-
-Traverse list of subscribed topics.
-Used to implement MQTT server when `MQTT_CMD_SUBSCRIBE` is received.
-Initial position `pos` should be 4.
-
-Parameters:
-- `mgs` - MQTT message
-- `topic` - Pointer to `mg_str` to receive topic
-- `qos` - Pointer to `uint8_t` to receive QoS
-- `pos` - Position to list from
-
-Return value: Next position, or 0 when done
-
-Usage example:
-
-```c
-// Mongoose events handler
-void fn(struct mg_connection *c, int ev, void *evd, void *fnd) {
-  if (ev == MG_EV_MQTT_CMD) {
-    struct mg_mqtt_message *mm = (struct mg_mqtt_message *) ev_data;
-    if (mm->cmd == MQTT_CMD_SUBSCRIBE) {
-      size_t pos = 4;
-      uint8_t qos;
-      struct mg_str topic;
-      // Iterate over all subscribed topics
-      while ((pos = mg_mqtt_next_sub(mm, &topic, &qos, pos)) > 0) {
-        LOG(LL_INFO, ("SUB [%.*s]", (int) topic.len, topic.ptr));
-      }
-    }
-  }
-}
-```
-
-### mg\_mqtt\_next\_unsub()
-
-```c
-size_t mg_mqtt_next_unsub(struct mg_mqtt_message *msg, struct mg_str *topic, size_t pos);
-```
-
-Same as `mg_mqtt_next_sub()`, but for unsubscribed topics. The difference
-is that there is no QoS in unsubscribe request.
-
-Parameters:
-- `mgs` - MQTT message
-- `topic` - Pointer to `mg_str` to receive topic
-- `pos` - Position from which to list 
-
-Return value: Next position, or 0 when done
-
-Usage example:
-
-```c
-// Mongoose events handler
-void fn(struct mg_connection *c, int ev, void *evd, void *fnd) {
-  if (ev == MG_EV_MQTT_CMD) {
-    struct mg_mqtt_message *mm = (struct mg_mqtt_message *) ev_data;
-    if (mm->cmd == MQTT_CMD_UNSUBSCRIBE) {
-      size_t pos = 4;
-      struct mg_str topic;
-      if (mm->cmd == MQTT_CMD_UNSUBSCRIBE) {
-        // Iterate over all unsubscribed topics
-        while ((pos = mg_mqtt_next_unsub(mm, &topic, pos)) > 0) {
-          LOG(LL_INFO, ("SUB [%.*s]", (int) topic.len, topic.ptr));
-        }
-      }
-   }
-  }
-}
-```
-
 ### mg\_mqtt\_send_header()
 
 ```c