address review comments

Author: japaric

src/SUMMARY.md

@@ -113,7 +113,6 @@
 
 - [Application Binary Interface](abi.md)
 - [Beneath `std`](beneath-std.md)
-  - [#[panic_implementation]](panic-implementation.md)
 
 [Appendix: Influences](influences.md)
 

src/beneath-std.md

@@ -1,10 +1,39 @@
 # Beneath `std`
 
-This section documents (or will document) features that are provided by the standard library and
-that `#![no_std]` developers have to deal with (i.e. provide) to build `#![no_std]` binary crates. A
-(likely incomplete) list of such features is shown below:
-
-- #[lang = "eh_personality"]
-- #[lang = "start"]
-- #[lang = "termination"]
-- #[panic_implementation]
+This section documents features that are provided by the standard library and that `#![no_std]`
+developers have to deal with (i.e. provide) to build `#![no_std]` binary crates. A list of such
+features is shown below:
+
+- `#[panic_implementation]`
+
+## `#[panic_implementation]`
+
+The `panic_implementation` attribute can only be applied to a function with signature
+`fn(&PanicInfo) -> !`. The function marked with this attribute defines the behavior of `panic!` in
+`#![no_std]` applications. The [`PanicInfo`] struct contains information about the location of the
+panic. There must be a single `panic_implementation` function in the dependency graph of a
+binary, dylib or cdylib crate.
+
+[`PanicInfo`]: https://doc.rust-lang.org/nightly/core/panic/struct.PanicInfo.html
+
+Below is shown a `panic_implementation` function that logs the panic message and then aborts the
+program.
+
+``` rust
+#![feature(core_intrinsics)]
+#![feature(panic_implementation)]
+#![no_std]
+
+use core::intrinsics;
+use core::panic::PanicInfo;
+
+#[panic_implementation]
+fn panic(info: &PanicInfo) -> ! {
+    let mut sink = /* .. */;
+
+    // logs "panicked at '$reason', src/main.rs:27:4" to some `sink`
+    let _ = writeln!(sink, "{}", info);
+
+    unsafe { intrinsics::abort() }
+}
+```