Update TempURL docs to include containers

Change-Id: Ifb7c26b23ec81e3bde96b6e3bad0e950c0ca9408

doc/source/api/form_post_middleware.rst

@@ -165,7 +165,8 @@ signature includes these elements from the form:
    is set to ``600`` seconds into the future.
 
 -  The secret key. Set as the ``X-Account-Meta-Temp-URL-Key`` header
-   value.
+   value for accounts or ``X-Container-Meta-Temp-URL-Key`` header
+   value for containers.  See :ref:`secret_keys` for more information.
 
 The following example code generates a signature for use with form
 **POST**:
@@ -214,4 +215,3 @@ This example uses the **swift-form-signature** script to compute the
            -F signature=35129416ebda2f1a21b3c2b8939850dfc63d8f43 \
            -F redirect=https://example.com/done.html \
            -F file=@flower.jpg
-

doc/source/api/temporary_url_middleware.rst

@@ -69,9 +69,8 @@ Secret Keys
 
 The cryptographic signature used in Temporary URLs and also in
 :doc:`form_post_middleware` uses a secret key. Object Storage allows you to
-store four secret key values. Two are stored at the account level and two
-are stored at the container level. When validating a request,
-Object Storage checks signatures against all keys. Using two keys at
+store two secret key values per account, and two per container. When validating
+a request, Object Storage checks signatures against all keys. Using two keys at
 each level enables key rotation without invalidating existing temporary URLs.
 
 To set the keys at the account level, set one or both of the following