sync: document behavior of Map.Delete when key is not present

Change-Id: I6a6a465631e91141a5fcc9c04a7df0dc47780731
Reviewed-on: https://go-review.googlesource.com/c/go/+/643955
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/sync/hashtriemap.go

@@ -74,6 +74,7 @@ func (m *Map) LoadAndDelete(key any) (value any, loaded bool) {
 }
 
 // Delete deletes the value for a key.
+// If the key is not in the map, Delete does nothing.
 func (m *Map) Delete(key any) {
 	m.m.Delete(key)
 }