Auto merge of rust-lang#6019 - rail-rain:improve_known_probs_mut_key, r=flip1995

Improve the "known problems" section of `interior_mutable_key`

* Remove the mention to `Rc` and `Arc` as these are `Freeze` (despite my intuition) so the lint correctly handles already.
* Instead, explain what could cause a false positive, and mention `bytes` as an example.

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changelog: Improved the "known problems" section of `interior_mutable_key`

Author: bors

clippy_lints/src/mut_key.rs

@@ -12,8 +12,10 @@ declare_clippy_lint! {
     /// `BtreeSet` rely on either the hash or the order of keys be unchanging,
     /// so having types with interior mutability is a bad idea.
     ///
-    /// **Known problems:** We don't currently account for `Rc` or `Arc`, so
-    /// this may yield false positives.
+    /// **Known problems:** It's correct to use a struct, that contains interior mutability
+    /// as a key, when its `Hash` implementation doesn't access any of the interior mutable types.
+    /// However, this lint is unable to recognize this, so it causes a false positive in theses cases.
+    /// The `bytes` crate is a great example of this.
     ///
     /// **Example:**
     /// ```rust