docs: specify s2n_blob growable conditions

[jmayclin]

Description of changes:

The growable field on s2n_blob does have a purpose, but it took me a lot of investigation to figure out what that purpose was. So I added a comment!

If we ever decide that zero-sized allocation are forbidden, we can remove it.

Testing:

All unit tests should continue to pass.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[lrstewart]

I'm not sure whether this behavior was actually intentional 🤔 Kind of seems like it was an artifact of aliasing "s2n_alloc" to "s2n_realloc", probably for unrelated reasons.

Separating "growable" from "allocated" definitely makes sense in the stuffer, because "allocated" indicates whether or not the data needs to be freed, and "growable" indicates whether insufficient space should be an error or lead to an allocation (usually it's an error).

But I guess with a blob, it wouldn't grow automatically, and you'd have to explicitly call "realloc", which is pretty strong indicator that you want to allocate more memory :)

[jmayclin]

Ya, I think "growable" is just a very confusing concept for the blob, and don't understand a scenario where

1. a stuffer would have growable=false
2. a blob would have growable=true
   or vice versa.

I opened #4944 to track further "normalization".