RFR(XS) 8212612 - Add documentation about Arguments::_exit_hook

[Ioi Lam]

http://cr.openjdk.java.net/~iklam/jdk12/8212612-exit-hook-docs.v01/
https://bugs.openjdk.java.net/browse/JDK-8212612

The following fields are assigned to according to 
JavaVMOption.optionString.

   abort_hook_t Arguments::_abort_hook = NULL;
   exit_hook_t Arguments::_exit_hook = NULL;
   vfprintf_hook_t Arguments::_vfprintf_hook = NULL;

However, there's no code in the JDK repo that uses these options.
Instead, they are intended for programs that embed the JVM using
JNI_CreateJavaVM.

We should add comments in arguments.cpp for clarification. Otherwise
this would look like dead code and someone might be tempted to
remove it.

Thanks
- Ioi