[+] Documentation for details/return_of.hpp

Signed-off-by: Jesse Klugmann <jesse@fd-imaging.com>
[+] Added documentation for details/conditional_type.hpp

Signed-off-by: Jesse Klugmann <jesse@fd-imaging.com>
[+] Added documentation for property.hpp

Signed-off-by: Jesse Klugmann <jesse@fd-imaging.com>