s/which/that/

Which is usually for descriptive clauses in technical writing, while
that is restrictive. Most of the time we are being being restrictive.
diff --git a/docs/api-stability.rst b/docs/api-stability.rst
index e87cc14..6a2e60e 100644
--- a/docs/api-stability.rst
+++ b/docs/api-stability.rst
@@ -7,7 +7,7 @@
 What does this policy cover?
 ----------------------------
 
-This policy includes any API or behavior which is documented in this
+This policy includes any API or behavior that is documented in this
 documentation.
 
 What does "stable" mean?
@@ -47,5 +47,5 @@
   ``DeprecationWarning``.
 * In ``cryptography X.Y+3`` the feature will be removed or changed.
 
-In short, code which runs without warnings will always continue to work for a
+In short, code that runs without warnings will always continue to work for a
 period of two releases.