`std::fmt` needs doc for `precision := '*'` case

[pnkfelix]

I spent a while looking at http://doc.rust-lang.org/nightly/std/fmt/index.html#syntax and comparing it against the rest of the API documentation there, but I saw no discussion of the meaning of the asterisk precision, e.g. what the format string "{:.*}" denotes.

The error message you get when you try to use it incorrectly is pretty inscrutable:

    println!("{:.*}", 0.0);

yields

<std macros>:3:11: 3:36 error: invalid reference to argument `1` (there is 1 argument)
<std macros>:3 print ! ( concat ! ( $ fmt , "\n" ) , $ ( $ arg ) * ) ) ;
                         ^~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of format_args!

---

Manual experimentation seems to indicate that the meaning of {:.*} is that the next two parameters are consumed, where the first represents the fractional precision and the second the value to be printed.

As a concrete example, the following four statements all print the same thing:

fn main() {
    println!("Hello {1} is {2:.0$}", 5, "x", 0.01);
    println!("Hello {0} is {2:.1$}", "x", 5, 0.01);
    println!("Hello {} is {:.*}",    "x", 5, 0.01);
    println!("Hello {} is {2:.*}",   "x", 5, 0.01);
}

namely:

Hello x is 0.01000
Hello x is 0.01000
Hello x is 0.01000
Hello x is 0.01000