gh-104717: Add comment about manual loop unrolling (gh-104718)

diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 7436c11..254cd9a 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -944,6 +944,7 @@ unicodekeys_lookup_unicode(PyDictKeysObject* dk, PyObject *key, Py_hash_t hash)
         }
         perturb >>= PERTURB_SHIFT;
         i = mask & (i*5 + perturb + 1);
+        // Manual loop unrolling
         ix = dictkeys_get_index(dk, i);
         if (ix >= 0) {
             PyDictUnicodeEntry *ep = &ep0[ix];