It is currently not possible to tell if a function or method is a `const fn` by reading documentation, as rustdoc does not retain the `const` keyword in `const fn` declarations

Basically, a function written for example like this:

    pub const fn add(a: i32, b: i32) -> i32 {
        a + b
    }

is currently documented like this:

`pub fn add(a: i32, b: i32) -> i32`

but it would be nice if was instead documented like this:

`pub const fn add(a: i32, b: i32) -> i32`