improve `t.throws` & `t.notThrows` docs
improve `t.throws` & `t.notThrows` docs

fixes #1120

Collinslenjo/ava,avajs/ava,sindresorhus/ava,andywer/ava-ts,avajs/ava,andywer/ava-ts,Collinslenjo/ava,BarryThePenguin/ava