Misleading documentation in `std::option` module

[jfrimmel]

The current std::option module states, that

Rust's pointer types must always point to a valid location; there are no "null" pointers.

Which is obviously wrong. There are no null references (&T), but there may be raw null pointers (*const T).
Maybe the docs should have used the terminology of smart pointers, which would be less misleading and be correct 😄

[steveklabnik]

I'd accept a PR to change "pointer" to "reference" here.

[srinivasreddy]

I think we can close this.