Update docs for fmt::write.

I reworded it slightly to make it more clear that the function only take
two arguments - the output and the Arguments struct that can be
generated from the format_args! macro.

r? @steveklabnik

src/libcore/fmt/mod.rs

@@ -897,14 +897,11 @@ pub trait UpperExp {
     fn fmt(&self, f: &mut Formatter) -> Result;
 }
 
-/// The `write` function takes an output stream, a precompiled format string,
-/// and a list of arguments. The arguments will be formatted according to the
-/// specified format string into the output stream provided.
+/// The `write` function takes an output stream, and an `Arguments` struct
+/// that can be precompiled with the `format_args!` macro.
 ///
-/// # Arguments
-///
-///   * output - the buffer to write output to
-///   * args - the precompiled arguments generated by `format_args!`
+/// The arguments will be formatted according to the specified format string
+/// into the output stream provided.
 ///
 /// # Examples
 ///