Expand permissions docs. Closes #2223.

1 files changed, 13 insertions, 1 deletions

diff --git a/docs/api-guide/permissions.md b/docs/api-guide/permissions.md
index ddcefadb..743ca435 100644
--- a/docs/api-guide/permissions.md
+++ b/docs/api-guide/permissions.md
@@ -10,12 +10,24 @@ Together with [authentication] and [throttling], permissions determine whether a
 
 Permission checks are always run at the very start of the view, before any other code is allowed to proceed.  Permission checks will typically use the authentication information in the `request.user` and `request.auth` properties to determine if the incoming request should be permitted.
 
+Permissions are used to grant or deny access different classes of users to different parts of the API.
+
+The simplest style of permission would be to allow access to any authenticated user, and deny access to any unauthenticated user. This corresponds the `IsAuthenticated` class in REST framework.
+
+A slightly less strict style of permission would be to allow full access to authenticated users, but allow read-only access to unauthenticated users. This corresponds to the `IsAuthenticatedOrReadOnly` class in REST framework.
+
 ## How permissions are determined
 
 Permissions in REST framework are always defined as a list of permission classes.
 
 Before running the main body of the view each permission in the list is checked.
-If any permission check fails an `exceptions.PermissionDenied` exception will be raised, and the main body of the view will not run.
+If any permission check fails an `exceptions.PermissionDenied` or `exceptions.NotAuthenticated` exception will be raised, and the main body of the view will not run.
+
+When the permissions checks fail either a "403 Forbidden" or a "401 Unauthorized" response will be returned, according to the following rules:
+
+* The request was successfully authenticated, but permission was denied. *— An HTTP 403 Forbidden response will be returned.*
+* The request was not successfully authenticated, and the highest priority authentication class *does not* use `WWW-Authenticate` headers. *— An HTTP 403 Forbidden response will be returned.*
+* The request was not successfully authenticated, and the highest priority authentication class *does* use `WWW-Authenticate` headers. *— An HTTP 401 Unauthorized response, with an appropriate `WWW-Authenticate` header will be returned.*
 
 ## Object level permissions