Document 429 handling generically

We only had a few mentions of 429 handling, now documenting our expectation
generically.

docs/reference/api-conventions.asciidoc

@@ -302,6 +302,14 @@ Content-Type: application/vnd.elasticsearch+json; compatible-with=7
 Accept: application/vnd.elasticsearch+json; compatible-with=7
 ----------------------------------------------------------------------
 
+[discrete]
+[[api-push-back]]
+=== `TOO_MANY_REQUESTS (429)` status code push back
+
+Elasticsearch may respond with a `TOO_MANY_REQUESTS (429)` status code, indicating that it is too busy right now to
+handle the request. When this happens, you should consider to retry the operation after a short delay, preferably
+with randomized exponential back-off.
+
 [discrete]
 [[api-url-access-control]]
 === URL-based access control