Add a useful comment about the `RawList` stable hash cache.

I tried removing it to see what happened. Not a good idea!

compiler/rustc_middle/src/ty/impls_ty.rs

diff --git a/compiler/rustc_middle/src/ty/impls_ty.rs b/compiler/rustc_middle/src/ty/impls_ty.rs
index ac45ce8..95a1a1b 100644
--- a/compiler/rustc_middle/src/ty/impls_ty.rs
+++ b/compiler/rustc_middle/src/ty/impls_ty.rs
@@ -20,6 +20,9 @@ impl<'a, 'tcx, H, T> HashStable<StableHashingContext<'a>> for &'tcx ty::list::Ra
     T: HashStable<StableHashingContext<'a>>,
 {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        // Note: this cache makes an *enormous* performance difference on certain benchmarks. E.g.
+        // without it, compiling `diesel-2.2.10` can be 74% slower, and compiling
+        // `deeply-nested-multi` can be ~4,000x slower(!)
         thread_local! {
             static CACHE: RefCell<FxHashMap<(*const (), HashingControls), Fingerprint>> =
                 RefCell::new(Default::default());