Add note on unexpected exceptions to CPython security policy (#1825)

Co-authored-by: Seth Larson <seth@python.org>

Author: StanFromIreland

security/policy.rst

@@ -45,6 +45,8 @@ triggerable with data inputs that are reasonably sized for the use case.
 Availability vulnerabilities must also demonstrate an "upward" change in posture
 for the attacker, rather than a "lateral" one.
 This is to avoid handling performance improvements as security vulnerabilities.
+Exceptions are an expected part of control flow when processing inputs,
+therefore crashes resulting from unhandled exceptions are not security vulnerabilities.
 
 Vulnerabilities in dependencies of Python (such as zlib, Tcl/Tk, or OpenSSL)
 are not vulnerabilities in Python unless Python's use of the dependency