improve docstring to describe PURE_PYTHON is an int

Co-authored-by: Dieter Maurer <[email protected]>

src/zope/security/checker.py

@@ -24,10 +24,10 @@
 (:func:`getCheckerForInstancesOf`, :func:`selectChecker`).
 
 This module is accelerated with a C implementation on CPython by
-default. If the environment variable ``PURE_PYTHON`` is set to ``1``
-before this module is imported, the C extensions will be bypassed and
-the reference Python implementations will be used. This can be helpful for
-debugging and tracing.
+default. If the environment variable ``PURE_PYTHON`` (converted to
+``int``) has a nonzero value before this module is imported, the C
+extensions will be bypassed and the reference Python implementations
+will be used. This can be helpful for debugging and tracing.
 
 Debugging Permissions Problems
 ==============================