Document token header in federation auth response

When reading the OS-FEDERATION API documentation, it is not clear how
to find the ID of the token when requesting either an unscoped or
scoped token. Token requests for the OS-FEDERATION API work the same
way as for the standard API, which is that the token ID is returned in
the X-Subject-Token header, so let's just mention that in the
OS-FEDERATION API documentation.

Change-Id: I6e9743d9001684f0d05ace119509f643c8b8e363

api-ref/source/v3-ext/federation/auth/auth.inc

@@ -40,6 +40,7 @@ Response
 
 .. rest_parameters:: federation/auth/parameters.yaml
 
+   - X-Subject-Token: X-Subject-Token
    - token: unscoped_token
 
 Response Example
@@ -81,6 +82,7 @@ Response
 
 .. rest_parameters:: federation/auth/parameters.yaml
 
+   - X-Subject-Token: X-Subject-Token
    - token: scoped_token
 
 Response Example

api-ref/source/v3-ext/federation/auth/parameters.yaml

@@ -1,5 +1,14 @@
 # variables in header
 
+X-Subject-Token:
+  description: |
+    The authentication token. An authentication
+    response returns the token ID in this header rather than in the
+    response body.
+  in: header
+  required: true
+  type: string
+
 # variables in path
 
 idp_id: