Add manage_own_api_key cluster privilege documentation (#465)

This commit documents manage_own_api_key cluster privilege.

Related to elastic/elasticsearch#45897 and elastic/elasticsearch#40031

Author: bizybot

docs/en/stack/security/authorization/privileges.asciidoc

@@ -71,6 +71,13 @@ roles of the user who created or updated them.
 
 --
 
+`manage_own_api_key`::
+All security-related operations on {es} API keys that are owned by the current
+authenticated user. The operations include 
+{ref}/security-api-create-api-key.html[creating new API keys],
+{ref}/security-api-get-api-key.html[retrieving information about API keys], and
+{ref}/security-api-invalidate-api-key.html[invalidating API keys].
+
 `manage_pipeline`::
 All operations on ingest pipelines.