Minor improvement to docs for rc::Rc

[lukaslueg]

The docs for try_unwrap() currently read

Unwraps the contained value if the Rc has only one strong reference.

If I read this correctly, what it is actually saying is

Unwraps the contained value if and only if the Rc has exactly one strong reference.

Is this what is says?

[steveklabnik]

Yes, that's correct. I'm writing a patch to clarify right now.