Add missing 63/64 rule documentation and a test (#462)

- Prominently point out the important differences of the `pallet-revive`
runtime.
- Add a test case illustrating the problem.

See also: https://github.com/paritytech/contract-issues/issues/257

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

docs/searcher-c2a407aa.js

@@ -437,7 +437,7 @@ window.search = window.search || {};
         if (yes) {
             loadSearchScript(
                 window.path_to_searchindex_js ||
-                path_to_root + 'searchindex-4ae832c1.js',
+                path_to_root + 'searchindex-c7304031.js',
                 'mdbook-search-index');
             search_wrap.classList.remove('hidden');
             searchicon.setAttribute('aria-expanded', 'true');