Least-Upper-Bound coercions don't document special behaviour of coercing to fnptrs

fn foo() {}
fn bar() {}

let a = match true {
    true => foo,
    false => bar,
};

Here we wind up with a LUB coercion between two different function item types. This succeeds by coercing both types to function pointers.

The current documentation for type coercions defines LUB coercions as simply trying one-to-one coercion both ways and then trying to compute a mutual supertype if that fails. It should also document that when we have a LUB coercion between function item types and closures we'll coerce to a function pointer if there's no mutual supertype.

Start in the Rust Reference section documenting type coercions and read the existing least-upper-bound coercion rules. Update the documentation to state how LUB coercions involving function items or closures fall back to function pointers when no mutual supertype exists, and ensure the provided example is covered by the explanation.