Merge pull request #578 from share/ping-pong-docs

📝 Add `connection.ping()` to the documentation

Author: alecgibson

docs/api/connection.md

@@ -229,3 +229,18 @@ The document **must** be of a [type]({{ site.baseurl }}{% link types/index.md %}
 Return value
 
 > A [`Presence`]({{ site.baseurl }}{% link api/presence.md %}) instance tied to the given document
+
+### ping()
+
+Send a short message to the server, which will respond with another short message, emitted on the connection as a
+`'pong'` event.
+
+```js
+connection.ping()
+connection.on('pong', () => {
+  // The server is still there
+})
+```
+
+{: .warn }
+Calling `ping()` when not connected will throw an error with code `ERR_CANNOT_PING_OFFLINE`.