Improve documentation of automatic_retries and retry_non_idempotent

Co-Authored-By: Karol Bucek <[email protected]>

Author: jsvd

docs/index.asciidoc

@@ -86,6 +86,9 @@ output plugins.
 
 How many times should the client retry a failing URL. We highly recommend NOT setting this value
 to zero if keepalive is enabled. Some servers incorrectly end keepalives early requiring a retry!
+Only IO related failures will be retried, such as connection timeouts and unreachable hosts.
+Valid but non 2xx HTTP responses will always be retried, regardless of the value of this setting,
+unless `retry_failed` is set.
 Note: if `retry_non_idempotent` is NOT set only GET, HEAD, PUT, DELETE, OPTIONS, and TRACE requests will be retried.
 
 [id="plugins-{type}s-{plugin}-cacert"]
@@ -316,6 +319,7 @@ Set this to false if you don't want this output to retry failed requests
   * Default value is `false`
 
 If `automatic_retries` is enabled this will cause non-idempotent HTTP verbs (such as POST) to be retried.
+This only affects connectivity related errors (see related `automatic_retries` setting).
 
 [id="plugins-{type}s-{plugin}-retryable_codes"]
 ===== `retryable_codes`