src/libcore/ptr.rs: Fix documentation for size of `Option<NonNull<T>>`
src/libcore/ptr.rs: Fix documentation for size of `Option<NonNull<T>>`

Seems more useful to say that it has the same size as `*mut T`.

aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust