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

@@ -20,6 +20,9 @@ where
     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());