Add a note explaining that no sub-repo perms = sub-repo perms disabled (#1090)

Just a small note clarifying that if no sub-repo permissions have been
set for a repository, that repository is treated as not having sub-repo
permissions enabled.

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Author: pjlast

docs/admin/permissions/api.mdx

@@ -99,6 +99,8 @@ You can call `setRepositoryPermissionsForUsers` repeatedly to set permissions fo
 
 ## Setting sub-repository permissions for users
 
+> NOTE: If a user has no sub-repo permissions set for a specific repository (assuming that they have general access to the repository), they will have access to the entire repository contents.
+
 Sourcegraph supports permissions on a per-file/directory basis. 
 
 To enable the sub-repo permissions API, add the following to the [site configuration](/admin/config/site_config):