Warn admin 60 may be removed from protected branch levels

doc/api/protected_branches.md

@@ -12,6 +12,10 @@ The access levels are defined in the `ProtectedRefAccess::ALLOWED_ACCESS_LEVELS`
 60 => Admin access
 ```
 
+**Note:** The admin access level (`60`) may be removed in a future release and
+should not be considered stable. We are currently evaluating restrictions to
+the Owner role as an alternative.
+
 ## List protected branches
 
 Gets a list of protected branches from a project.