Usage Guide statement about memory usage could be more accurate

[maddeleine]

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Problem:

The Usage Guide states that the s2n_connection struct is 4KB. That is technically correct; it's what you'll get if you do sizeof(s2n_connection), but it's not really that useful. It doesn't take into consideration the pointers on the connection which may point to alloced memory. So the total memory used for the connection is larger than 4KB. We've had several PRs open to try to pin down how much memory s2n uses per connection but they haven't really answered the question completely and also haven't been prioritized.

Solution:

Ideally we would be able to give users an accurate picture of how much memory will be used per TLS connection. I don't know if this would be a doc or a test, maybe both, but we should have a better answer when people ask this question.