diff --git a/src/runtime/metrics/description.go b/src/runtime/metrics/description.go
index aea51c7f75..745691b24f 100644
--- a/src/runtime/metrics/description.go
+++ b/src/runtime/metrics/description.go
@@ -339,9 +339,11 @@ var allDesc = []Description{
 		Kind: KindUint64,
 	},
 	{
-		Name:        "/memory/classes/heap/stacks:bytes",
-		Description: "Memory allocated from the heap that is reserved for stack space, whether or not it is currently in-use.",
-		Kind:        KindUint64,
+		Name: "/memory/classes/heap/stacks:bytes",
+		Description: "Memory allocated from the heap that is reserved for stack space, whether or not it is currently in-use. " +
+			"Currently, this represents all stack memory for goroutines. It also includes all OS thread stacks in non-cgo programs. " +
+			"Note that stacks may be allocated differently in the future, and this may change.",
+		Kind: KindUint64,
 	},
 	{
 		Name:        "/memory/classes/heap/unused:bytes",
@@ -374,9 +376,13 @@ var allDesc = []Description{
 		Kind:        KindUint64,
 	},
 	{
-		Name:        "/memory/classes/os-stacks:bytes",
-		Description: "Stack memory allocated by the underlying operating system.",
-		Kind:        KindUint64,
+		Name: "/memory/classes/os-stacks:bytes",
+		Description: "Stack memory allocated by the underlying operating system. " +
+			"In non-cgo programs this metric is currently zero. This may change in the future." +
+			"In cgo programs this metric includes OS thread stacks allocated directly from the OS. " +
+			"Currently, this only accounts for one stack in c-shared and c-archive build modes, " +
+			"and other sources of stacks from the OS are not measured. This too may change in the future.",
+		Kind: KindUint64,
 	},
 	{
 		Name:        "/memory/classes/other:bytes",
diff --git a/src/runtime/metrics/doc.go b/src/runtime/metrics/doc.go
index 5238bcea8e..5c52f78477 100644
--- a/src/runtime/metrics/doc.go
+++ b/src/runtime/metrics/doc.go
@@ -318,7 +318,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 
 	/memory/classes/heap/stacks:bytes
 		Memory allocated from the heap that is reserved for stack space,
-		whether or not it is currently in-use.
+		whether or not it is currently in-use. Currently, this
+		represents all stack memory for goroutines. It also includes all
+		OS thread stacks in non-cgo programs. Note that stacks may be
+		allocated differently in the future, and this may change.
 
 	/memory/classes/heap/unused:bytes
 		Memory that is reserved for heap objects but is not currently
@@ -345,6 +348,12 @@ Below is the full list of supported metrics, ordered lexicographically.
 
 	/memory/classes/os-stacks:bytes
 		Stack memory allocated by the underlying operating system.
+		In non-cgo programs this metric is currently zero. This may
+		change in the future.In cgo programs this metric includes
+		OS thread stacks allocated directly from the OS. Currently,
+		this only accounts for one stack in c-shared and c-archive build
+		modes, and other sources of stacks from the OS are not measured.
+		This too may change in the future.
 
 	/memory/classes/other:bytes
 		Memory used by execution trace buffers, structures for debugging
diff --git a/src/runtime/mstats.go b/src/runtime/mstats.go
index 9a247b87b5..9cdc565137 100644
--- a/src/runtime/mstats.go
+++ b/src/runtime/mstats.go
@@ -199,7 +199,17 @@ type MemStats struct {
 	// StackSys is bytes of stack memory obtained from the OS.
 	//
 	// StackSys is StackInuse, plus any memory obtained directly
-	// from the OS for OS thread stacks (which should be minimal).
+	// from the OS for OS thread stacks.
+	//
+	// In non-cgo programs this metric is currently equal to StackInuse
+	// (but this should not be relied upon, and the value may change in
+	// the future).
+	//
+	// In cgo programs this metric includes OS thread stacks allocated
+	// directly from the OS. Currently, this only accounts for one stack in
+	// c-shared and c-archive build modes and other sources of stacks from
+	// the OS (notably, any allocated by C code) are not currently measured.
+	// Note this too may change in the future.
 	StackSys uint64
 
 	// Off-heap memory statistics.