Atexit handler and other auto-cleanup behavior is not documented

[maddeleine]

Security issue notifications

If you discover a potential security issue in s2n we ask that you notify
AWS Security via our vulnerability reporting page. Please do not create a public github issue.

Problem:

The automatic cleanup part of our code is super confusing to read and also is not documented. This leads to any cleanup issue taking a lot longer than it should since no one remembers how this code works.

Solution:

Minimally, we should update the development guide to explain how s2n deals with automatic cleanup of memory. Ideally we would fix the code so it is less confusing.

Requirements / Acceptance Criteria:

We should document in the development guide our autocleanup behavior.