Merge pull request #124 from ycombinator/backport/6.x/pr-80_pr-80

[6.x] Documents built-in `beats_admin` role | Clarifying purpose of index

Author: ycombinator

docs/en/stack/security/authorization/built-in-roles.asciidoc

@@ -14,6 +14,10 @@ to users. These roles have a fixed set of privileges and cannot be updated.
 Grants access necessary for the APM system user to send system-level data
 (such as monitoring) to {es}.
 
+[[built-in-roles-beats-admin]] `beats_admin` ::
+Grants access to the `.management-beats` index, which contains configuration
+information for the Beats.
+
 [[built-in-roles-beats-system]] `beats_system` ::
 Grants access necessary for the Beats system user to send system-level data
 (such as monitoring) to {es}.