7179567: JCK8 tests: api/java_net/URLClassLoader/index.html#Ctor3 failed with NPE
6445180: URLClassLoader does not describe the behavior of several methods with respect to null arguments
Summary: Document when a NPE will be thrown by URLClassLoader constructors, newInstance(), findClass(), and getPermissions().
Reviewed-by: alanb, mduigou, chegar, dholmes, jrose