Document `t.try()`

[novemberborn]

#1947 landed a new t.try() assertion. It's experimental, one of the reasons for which is that we don't have documentation yet.

We need to document the assertion, how to use it and what the edge cases are.

Apologies for how bare this issue is — happy to elaborate more if you're looking to contribute this 😍

[novemberborn]

See also avajs/eslint-plugin-ava#273 for a discussion on what the argument for the callback function should be named.

[novemberborn]

@qlonik shared their initial stab at the documentation here: #2251