Change the first line of the println macro doc

This makes the first lines of the print! and println! macros
different. Previously, they would show up exactly the same in the
documentation for the macros in libstd [1], with nothing about how
one of them also prints a newline.

[1]: https://doc.rust-lang.org/stable/std/#macros

src/libstd/macros.rs

@@ -98,7 +98,7 @@ macro_rules! print {
     ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
 }
 
-/// Macro for printing to the standard output.
+/// Macro for printing to the standard output, with a newline.
 ///
 /// Use the `format!` syntax to write data to the standard output.
 /// See `std::fmt` for more information.