document that ava assertions don't throw

I didn't realize until recently that ava assertions don't stop the test when they fail.

I think this could be documented better, and I'm willing to do so if that's useful

[novemberborn]

I thought it was referenced, but I can't immediately spot it in the obvious places. This hints at it:

If multiple assertion failures are encountered within a single test, AVA will only display the first one.

(From https://github.com/avajs/ava/blob/master/docs/03-assertions.md).

That might be the best spot to add a clarification?

I'd scatter reinforcement of the idea at several places, since reference documents are often not read linearly, and "assert doesn't throw" is pretty unusual; in almost every other programming context, assert does throw.

I can make a PR in a day or so.

[novemberborn]

@f5lee fantastic, thank you.

[earthlyreason]

For reference, see also #1485