Document keyword variable of TokenAuthentication

Author: hroncok

docs/api-guide/authentication.md

@@ -143,6 +143,8 @@ For clients to authenticate, the token key should be included in the `Authorizat
 
     Authorization: Token 9944b09199c62bcf9418ad846dd0e4bbdfc6ee4b
 
+**Note:** If you want to use a different keyword in the header, such as `Bearer`, simply subclass `TokenAuthentication` and set the `keyword` class variable.
+
 If successfully authenticated, `TokenAuthentication` provides the following credentials.
 
 * `request.user` will be a Django `User` instance.