Document impl Clone for Rc with Rc specific information

[jakerr]

The current doc for Rc has:

• impl Clone for Rc
  • fn clone(&self) -> Rc
    • Returns a copy of the value. The contents of owned pointers are copied to maintain uniqueness, while the contents of managed pointers are not copied.

As the above doesn't make sense for Rc it should have a specific comment.

[huonw]

cc #11991

[steveklabnik]

This doesn't say it on the Rc<T> page, but if you click through to the trait page, does say it there: http://doc.rust-lang.org/std/clone/trait.Clone.html#tymethod.clone

[steveklabnik]

This will be fixed by #19112