When/how to use asserts

There's a really good point in [Style.md](https://github.com/lefticus/cppbestpractices/blob/master/03-Style.md) on how not to use asserts, but should we document when to use them as well? I think the c++ usage is quite different to other languages, where asserts are used for test code but not for dev code.

Read the existing guidance in Style.md and review the issue discussion about assert usage. Clarify the intended C++ guidance for when asserts are appropriate, then document the agreed recommendation alongside the existing advice on when not to use them.

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