[DOCS] Note circuit breakers reject requests with 429 HTTP status code (#69864)

We mention Elasticsearch returns 429 if the circuit breaker trips in https://www.elastic.co/blog/improving-node-resiliency-with-the-real-memory-circuit-breaker, but there is no mention in the docs.

This adds an xref to circuit breaker errors section.

Co-authored-by: James Rodewig <40268737+jrodewig@users.noreply.github.com>

docs/reference/how-to/fix-common-cluster-issues.asciidoc

@@ -21,7 +21,8 @@ memory pressure if usage consistently exceeds 85%.
 
 **Error messages**
 
-If a request triggers a circuit breaker, {es} returns an error.
+If a request triggers a circuit breaker, {es} returns an error with a `429` HTTP
+status code.
 
 [source,js]
 ----

docs/reference/modules/indices/circuit_breaker.asciidoc

@@ -8,6 +8,8 @@ Except where noted otherwise, these settings can be dynamically updated on a
 live cluster with the <<cluster-update-settings,cluster-update-settings>> API.
 // end::circuit-breaker-description-tag[]
 
+For information about circuit breaker errors, see <<circuit-breaker-errors>>.
+
 [[parent-circuit-breaker]]
 [discrete]
 ==== Parent circuit breaker