Document how DataContainer can store entries in different format than provided to Cache

Off-heap container entries contain some additional bytes (ASCII 1,2,1,4), which probably shouldn't be there. Besides additional consumption of memory, it can cause problem as entry put into the cache with cache API is not accessible via container API (which is available to the use), i.e. something like this will fail:

cache.put(key, value);
container = AbstractDelegatingCache.unwrapCache(cache).getAdvancedCache().getDataContainer();
InternalCacheEntry entry = container.get(key);