Include output stream in panic!() documentation

Author: jfrimmel

library/core/src/macros/panic.md

@@ -10,8 +10,8 @@ tests. `panic!` is closely tied with the `unwrap` method of both
 `panic!` when they are set to [`None`] or [`Err`] variants.
 
 This macro is used to inject panic into a Rust thread, causing the thread to
-panic entirely. This macro panics with a string and uses the [`format!`] syntax
-for building the message.
+panic entirely. The string built by this macro (using the [`format!`] syntax
+for building the actual message) is printed to `stderr`.
 
 Each thread's panic can be reaped as the [`Box`]`<`[`Any`]`>` type,
 which contains either a `&str` or `String` for regular `panic!()` invocations.