runtime: fix stale comment for mheap/malloc

mheap use pageAlloc to manage free/scav address space instead of
using free/scav treap. The comment on mheap states mheap uses
treaps. Update the comment to reflect the use of pageAlloc.

In the fallback code when sizeSpecializedMalloc is enabled,
heapBitsInSpan is false. Update the comment to reflect that.

Change-Id: I50d2993c84e2c0312a925ab0a33065cc4cd41c41
Reviewed-on: https://go-review.googlesource.com/c/go/+/722700
Reviewed-by: Lidong Yan <yldhome2d2@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Mark Freeman <markfreeman@google.com>

Author: brandb97

src/runtime/malloc.go

@@ -1169,7 +1169,7 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 	var x unsafe.Pointer
 	var elemsize uintptr
 	if sizeSpecializedMallocEnabled {
-		// we know that heapBitsInSpan is true.
+		// we know that heapBitsInSpan is false.
 		if size <= maxSmallSize-gc.MallocHeaderSize {
 			if typ == nil || !typ.Pointers() {
 				x, elemsize = mallocgcSmallNoscan(size, typ, needzero)

src/runtime/mheap.go

@@ -56,7 +56,7 @@ const (
 )
 
 // Main malloc heap.
-// The heap itself is the "free" and "scav" treaps,
+// The heap use pageAlloc to manage free and scavenged pages,
 // but all the other global data is here too.
 //
 // mheap must not be heap-allocated because it contains mSpanLists,