Add comment to explain the implications of not sorting keywords (#3331)

In Python 3.6, sorted() was removed from _make_key() for the lru_cache and instead rely on guaranteed keyword argument order preservation.  This makes keyword argument handling faster but it also causes multiple callers with a different keyword argument order to be cached as separate items.  Depending on your point of view, this is either a performance regression (increased number of cache misses) or a performance enhancement (faster computation of keys).