RC module documentation concerning immutability is misleading

[justintaft]

RC module documentation should clarify mutable references can be obtained, under certain conditions.

Rc module documentation sates:
"Shared references in Rust disallow mutation by default, and Rc is no exception: you cannot obtain a mutable reference to something inside an Rc."

https://doc.rust-lang.org/std/rc/

Rc's get_mut method states:
"Returns a mutable reference to the inner value, if there are no other Rc or Weak pointers to the same value.".

https://doc.rust-lang.org/std/rc/struct.Rc.html#method.get_mut