Document #[repr(u8)] enum to enable LLVM optimizations

See rust-lang/rust#27303 and rust-lang/libc#48 for discussion of the
optimization, or just try compiling `unsafe {free(malloc(16))}` with various
Rust definitions of `void *` in the FFI binding.

src/other-reprs.md

@@ -50,6 +50,15 @@ same discriminant.
 On non-C-like enums, this will inhibit certain optimizations like the null-
 pointer optimization.
 
+For C-like enums, these may also be useful to enable certain LLVM optimizations.
+For instance, LLVM will recognize extern functions named `malloc` and `free` and
+optimize away unnecessary calls, as long as the functions are of the right type:
+`malloc` needs to return a C `void *`, which LLVM interprets as a pointer to
+`u8`, and `free` needs to accept one. The libc crate uses `#[repr(u8)]` on its
+`c_void` type to enable this optimization.  However, note that this optimization
+stops working if the enum is non-C-like, even if the only data is `()` or a
+`PhantomData`!
+
 These reprs have no effect on a struct.