reword note on explicitly clearing local references to pytest.raises() results

Made it clearer that clearing such references is not mandatory and is only an
optional step which may help the Python interpreter speed up its garbage
collection.

--HG--
branch : document_ExceptionInfo_ref_cycle

_pytest/python.py

@@ -938,12 +938,6 @@ def raises(ExpectedException, *args, **kwargs):
 
     This helper produces a ``py.code.ExceptionInfo()`` object.
 
-    Note that any local references made to such returned
-    ``py.code.ExceptionInfo()`` objects should be explicitly cleared, as they
-    are part of a reference cycle similar to local references to caught Python
-    exception objects. See the official Python ``try`` statement documentation
-    for more detailed information.
-
     If using Python 2.5 or above, you may use this function as a
     context manager::
 
@@ -968,6 +962,22 @@ def raises(ExpectedException, *args, **kwargs):
 
         >>> raises(ZeroDivisionError, "f(0)")
         <ExceptionInfo ...>
+
+    Performance note:
+    -----------------
+
+    Similar to caught exception objects in Python, explicitly clearing local
+    references to returned ``py.code.ExceptionInfo`` objects can help the Python
+    interpreter speed up its garbage collection.
+
+    Clearing those references breaks a reference cycle (``ExceptionInfo`` -->
+    caught exception --> frame stack raising the exception --> current frame
+    stack --> local variables --> ``ExceptionInfo``) which makes Python keep all
+    objects referenced from that cycle (including all local variables in the
+    current frame) alive until the next cyclic garbage collection run. See the
+    official Python ``try`` statement documentation for more detailed
+    information.
+
     """
     __tracebackhide__ = True
     if ExpectedException is AssertionError: