Add missing documentation for secretutils

The current doc page for secretutils is blank [1], most likely because
of the way constant_time_compare is in the exception block.

This patch manually tells sphinx to doc the constant_time_compare
function. Also adds the oslo.utils version when the module was added.

[1]: https://docs.openstack.org/developer/oslo.utils/api/secretutils.html

Change-Id: Ic1fb15176c93f126826f568149cabf4b8e5450d8

doc/source/api/secretutils.rst

@@ -3,4 +3,4 @@
 =============
 
 .. automodule:: oslo_utils.secretutils
-   :members:
+   :members: constant_time_compare

oslo_utils/secretutils.py

@@ -12,6 +12,12 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+"""
+Secret utilities.
+
+.. versionadded:: 3.5
+"""
+
 import hmac