runtime: fix mmap comments
We only pass lower 32 bits of file offset to asm routine.
R=r, dave, rsc
CC=golang-dev
https://golang.org/cl/6499118
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 96da29a..37427fb 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -728,6 +728,9 @@
/*
* low level C-called
*/
+// for mmap, we only pass the lower 32 bits of file offset to the
+// assembly routine; the higher bits (if required), should be provided
+// by the assembly routine as 0.
uint8* runtime·mmap(byte*, uintptr, int32, int32, int32, uint32);
void runtime·munmap(byte*, uintptr);
void runtime·madvise(byte*, uintptr, int32);
