copy constructors need documentation

[Manu]

Someone needs to write up the documentation for copy constructors.
There's no source on how to write them, and explaining how to handle
all the different qualifier cases and such.
Currently, you have to troll through threads and try and interpret a
DIP to work it out... it's not good enough. A user wouldn't even know
D has copy constructors by looking at the language reference on
dlang.org.