runtime/metrics: document runtime-internal locks in /sync/mutex/wait/total:seconds

For #57071.

Change-Id: I7ce6c35bed95a6ea3cdc17007f861c5dd82404d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/547056
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Rhys Hiltner <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: prattmic

Diff for: src/runtime/metrics/description.go

@@ -441,7 +441,7 @@ var allDesc = []Description{
 	},
 	{
 		Name:        "/sync/mutex/wait/total:seconds",
-		Description: "Approximate cumulative time goroutines have spent blocked on a sync.Mutex or sync.RWMutex. This metric is useful for identifying global changes in lock contention. Collect a mutex or block profile using the runtime/pprof package for more detailed contention data.",
+		Description: "Approximate cumulative time goroutines have spent blocked on a sync.Mutex, sync.RWMutex, or runtime-internal lock. This metric is useful for identifying global changes in lock contention. Collect a mutex or block profile using the runtime/pprof package for more detailed contention data.",
 		Kind:        KindFloat64,
 		Cumulative:  true,
 	},

Diff for: src/runtime/metrics/doc.go

@@ -447,10 +447,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 		monotonically.
 
 	/sync/mutex/wait/total:seconds
-		Approximate cumulative time goroutines have spent blocked
-		on a sync.Mutex or sync.RWMutex. This metric is useful for
-		identifying global changes in lock contention. Collect a mutex
-		or block profile using the runtime/pprof package for more
-		detailed contention data.
+		Approximate cumulative time goroutines have spent blocked on a
+		sync.Mutex, sync.RWMutex, or runtime-internal lock. This metric
+		is useful for identifying global changes in lock contention.
+		Collect a mutex or block profile using the runtime/pprof package
+		for more detailed contention data.
 */
 package metrics