Document reasoning behind vm.expectRevert error and when it is safe

[zerosnacks]

We should not refer to it as strongly discouraged but explain the implications in the docs and how it is a footgun.

One model of reference is the "memory-safe" annotation. If you annotate (i.e. allow internal reverts), you should understand what you’re doing, otherwise it can lead to undefined behaviour.

In short, heavier on content and less on the warning