x/sync/errgroup: make note of the default errgroup goroutine limit (unlimited)

Change-Id: I1926e6faf821a7adb6c1365d18432abb0f856367
Reviewed-on: https://go-review.googlesource.com/c/sync/+/409774
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Changkun Ou <mail@changkun.de>
Run-TryBot: Changkun Ou <mail@changkun.de>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: jeanbza

errgroup/errgroup.go

@@ -17,7 +17,8 @@ type token struct{}
 // A Group is a collection of goroutines working on subtasks that are part of
 // the same overall task.
 //
-// A zero Group is valid and does not cancel on error.
+// A zero Group is valid, has no limit on the number of active goroutines,
+// and does not cancel on error.
 type Group struct {
 	cancel func()