added panic docs for print\! and println\! macros

src/libstd/macros.rs

@@ -68,6 +68,10 @@ macro_rules! panic {
 /// necessary to use `io::stdout().flush()` to ensure the output is emitted
 /// immediately.
 ///
+/// # Panics
+///
+/// Panics if writing to `io::stdout()` fails.
+///
 /// # Examples
 ///
 /// ```
@@ -99,6 +103,10 @@ macro_rules! print {
 /// Use the `format!` syntax to write data to the standard output.
 /// See `std::fmt` for more information.
 ///
+/// # Panics
+///
+/// Panics if writing to `io::stdout()` fails.
+///
 /// # Examples
 ///
 /// ```