[review] revisit/document the rationale for qualified_parent_class_of design

[badair]

from http://lists.boost.org/Archives/boost/2017/04/234173.php

[ldionne]

Also seen in https://lists.boost.org/Archives/boost/2017/04/234333.php:

1. Why does qualified_parent_class_of return a ref qualified type even for unqualified PMFs? This is again surprising, I would expect qualified_parent_class_of_t<void (foo::*)()> to return foo, not foo&.
2. Why does qualified_parent_class_of always return a const& qualified type for PMDs? I can imagine that this would be quite an annoyance if the user wants to apply custom qualifiers to it for one reason or another, especially rvalue reference, which would be silently collapsed away by the lvalue reference. The safest choice IMO would be to return an unqualified type.