move the Valgrind doc-comments to Rust-land, where they actually matter

src/valgrind.c

@@ -1,22 +1,12 @@
 #include <stdint.h>
 #include "valgrind/valgrind.h"
 
-// In order for Valgrind to keep track of stack overflows and such, it needs
-// a little help. That help unfortunately comes in the form of a pair of C
-// macros. Calling out to un-inlineable C code for this is pointlessly slow,
-// but that's the way it is for now.
-
 typedef uint32_t valgrind_stack_id_t;
 
-// Register a stack with Valgrind. Returns an integer ID that can
-// be used to deregister the stack when it's deallocated.
-// `start < end`, though Valgrind will happily accept either.
 valgrind_stack_id_t valgrind_stack_register(const void *start, const void *end) {
   return VALGRIND_STACK_REGISTER(start, end);
 }
 
-// Deregister a stack from Valgrind. Takes the integer ID that was returned
-// on registration.
 void valgrind_stack_deregister(valgrind_stack_id_t id) {
   VALGRIND_STACK_DEREGISTER(id);
 }

src/valgrind.rs

@@ -1,9 +1,19 @@
 #![allow(non_camel_case_types)]
+//! In order for Valgrind to keep track of stack overflows and such, it needs
+//! a little help. That help unfortunately comes in the form of a pair of C
+//! macros. Calling out to un-inlineable C code for this is pointlessly slow,
+//! but that's the way it is for now.
 
 pub type stack_id_t = u32;
 extern "C" {
   #[link_name = "valgrind_stack_register"]
+  /// Register a stack with Valgrind. Returns an integer ID that can
+  /// be used to deregister the stack when it's deallocated.
+  /// `start < end`, though Valgrind will happily accept either.
   pub fn stack_register(start: *const u8, end: *const u8) -> stack_id_t;
+
   #[link_name = "valgrind_stack_deregister"]
+  /// Deregister a stack from Valgrind. Takes the integer ID that was returned
+  /// on registration.
   pub fn stack_deregister(id: stack_id_t);
 }