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`.

library/core/src/result.rs

@@ -1009,6 +1009,15 @@ impl<T, E> Result<T, E> {
 
     /// Returns the contained [`Ok`] value, consuming the `self` value.
     ///
+    /// Because this function may panic, its use is generally discouraged.
+    /// Instead, prefer to use pattern matching and handle the [`Err`]
+    /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
+    /// [`unwrap_or_default`].
+    ///
+    /// [`unwrap_or`]: Result::unwrap_or
+    /// [`unwrap_or_else`]: Result::unwrap_or_else
+    /// [`unwrap_or_default`]: Result::unwrap_or_default
+    ///
     /// # Panics
     ///
     /// Panics if the value is an [`Err`], with a panic message including the