Document `Option<extern "C" fn>` in the FFI tutorial

[nikomatsakis]

The FFI tutorial should reflect the fact that:

1. Option<thin-ptr-type> is guaranteed to be a pointer with None represented as NULL.
2. Option<extern "C" fn> is the recommended way to represent a nullable fn pointer.

[nikomatsakis]

Nominating for milestone.

[catamorphism]

Accepted for well-defined

[pnkfelix]

part of #11755

[brson]

Nominating to remove from milestone.

[pnkfelix]

We can ship 1.0 without this, though it would be good to have this. Leaving as P-high, but removing from 1.0 milestone.

[emberian]

Taking this.