Document "manage_api_key" cluster privilege

Relates: elastic/elasticsearch#43728

Author: tvernum

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

@@ -22,6 +22,14 @@ This includes snapshotting, updating settings, and rerouting. It also includes
 obtaining snapshot and restore status. This privilege does not include the 
 ability to manage security.
 
+`manage_api_key`::
+All security-related operations on {es} API Keys, including 
+{ref}/security-api-create-api-key.html[creating new API keys] (these will always
+be owned by the authenticated user) as well as 
+{ref}/security-api-get-api-key.html[retrieving information about API keys] and
+{ref}/security-api-invalidate-api-key.html[invalidating API keys] 
+(including those owned by other users).
+
 `manage_ccr`::
 All {ccr} operations related to managing follower indices and auto-follow 
 patterns. It also includes the authority to grant the privileges necessary to