Document 429 handling generically (#105700)

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

Co-authored-by: David Turner <david.turner@elastic.co>
Co-authored-by: Liam Thompson <32779855+leemthompo@users.noreply.github.com>

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]]
+=== HTTP `429 Too Many Requests` status code push back
+
+{es} APIs may respond with the HTTP `429 Too Many Requests` status code, indicating that the cluster is too busy
+to handle the request. When this happens, consider retrying after a short delay. If the retry also receives
+a `429 Too Many Requests` response, extend the delay by backing off exponentially before each subsequent retry.
+
 [discrete]
 [[api-url-access-control]]
 === URL-based access control