Fix undocumented functions in secretutils

Change-Id: Ic226f4b64b91a1a391d6be2cb3c9f781440ef318

doc/source/reference/secretutils.rst

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

oslo_utils/secretutils.py

@@ -31,6 +31,14 @@ import debtcollector.removals
 @debtcollector.removals.remove(message='Use hmac.compare_digest instead',
                                category=DeprecationWarning)
 def constant_time_compare(*args, **kwargs):
+    """Return True if both string or binary inputs are equal, otherwise False.
+
+    This function should take a constant amount of time regardless of
+    how many characters in the strings match. This function uses an
+    approach designed to prevent timing analysis by avoiding
+    content-based short circuiting behaviour, making it appropriate
+    for cryptography.
+    """
     return hmac.compare_digest(*args, **kwargs)