Rollup merge of #99579 - CleanCut:expect-warning, r=joshtriplett

Rollup merge of #99579 - CleanCut:expect-warning, r=joshtriplett

Add same warning to Result::expect as Result::unwrap

I was reading a recent blog post by Jimmy Hartzell and [he noted](https://www.thecodedmessage.com/posts/2022-07-14-programming-unwrap/#context):

> I will however note that the documentation of `unwrap` comes with [a warning not to use it](https://doc.rust-lang.org/std/result/enum.Result.html#method.unwrap). The warning is framed in terms of the fact that `unwrap` may panic, but the [documentation of `expect`](https://doc.rust-lang.org/std/result/enum.Result.html#method.expect), where this is equally true, does not come with such a warning.

It _is_ equally true. Let's add the same warning to `expect`. This PR is a copy-and-paste of the warning text from the docstring for `unwrap`.