add short docs about shouldNotReceive

docs/reference/expectations.rst

@@ -41,6 +41,12 @@ a set of operations which are recorded as expectations on the partial mock. A
 simple use case is automatically recording expectations based on an existing
 usage (e.g. during refactoring). See examples in a later section.
 
+.. code-block:: php
+
+    shouldNotReceive(method_name)
+
+Declares that the mock should not expect a call to the given method name.
+
 .. code-block:: php
 
     with(arg1, arg2, ...) / withArgs(array(arg1, arg2, ...))