[7.14] Document the Elasticsearch product check

Author: sethmlarson

docs/sphinx/transports.rst

@@ -21,6 +21,27 @@ and lightweight than the optional ``requests``-based class. Only use
 ``RequestsHttpConnection`` if you have need of any of ``requests`` advanced
 features like custom auth plugins etc.
 
+Product check on first request
+------------------------------
+
+Starting in v7.14.0 the client performs a required product check before
+the first API call is executed. This product check allows the client to
+establish that it's communicating with a valid Elasticsearch cluster.
+
+The product check requires a single HTTP request to the ``info`` API. In
+most cases this request will succeed quickly and then no further product
+check HTTP requests will be sent.
+
+The product check will verify that the ``X-Elastic-Product: Elasticsearch``
+HTTP header is being sent or if the ``info`` API response has proper values
+of ``tagline`` and ``version.build_flavor``.
+
+If the client detects that it's not connected to Elasticsearch the
+``NotElasticsearchError`` exception will be raised. In previous versions
+of Elasticsearch the ``info`` API required additional permissions so
+if an authentication or authorization error is raised during the
+product check then an ``ElasticsearchWarning`` is raised and the client
+proceeds normally.
 
 .. py:module:: elasticsearch.connection