runtime: improve godoc formatting of memclrNoHeapPointers comment

Fixes #28955

Change-Id: I738ad0c76f7bf8fc504a48cf55d3becd5ed7a9d6
Reviewed-on: https://go-review.googlesource.com/c/151337Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/stubs.go

@@ -68,12 +68,12 @@ func badsystemstack() {
 // used only when the caller knows that *ptr contains no heap pointers
 // because either:
 //
-// 1. *ptr is initialized memory and its type is pointer-free.
+// *ptr is initialized memory and its type is pointer-free, or
 //
-// 2. *ptr is uninitialized memory (e.g., memory that's being reused
+// *ptr is uninitialized memory (e.g., memory that's being reused
 // for a new allocation) and hence contains only "junk".
 //
-// in memclr_*.s
+// The (CPU-specific) implementations of this function are in memclr_*.s.
 //go:noescape
 func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)