Add some explanations as to why exception re-entrancy may still be an issue in a multicore-environment.

[kiarie404]

This PR attempts to add explanations on :

• why exceptions that contain mutable static variables are still considered re-entrant.
• why that 'assumed re-entrancy' can be challenged in a multicore environment

PS > possible improvements to this PR will be to :

• add example code to demonstrate how to ensure re-entrancy in a multicore environment.

[9names]

@eldruin did you forget to add this to the merge queue or are you waiting for something else to occur before doing that?

[jamesmunns]

It seems to have gotten stuck somehow, repoked it, hopefully it'll get merged now. Thanks for checking in @9names!