errgroup: document calling Go before Wait

Fixes golang/go#70284

Change-Id: I31d60ea182226c032b0ffbddcbb5b53675dfd5e6
Reviewed-on: https://go-review.googlesource.com/c/sync/+/660075
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

errgroup/errgroup.go

@@ -18,7 +18,7 @@ import (
 type token struct{}
 
 // A Group is a collection of goroutines working on subtasks that are part of
-// the same overall task.
+// the same overall task. A Group should not be reused for different tasks.
 //
 // A zero Group is valid, has no limit on the number of active goroutines,
 // and does not cancel on error.
@@ -61,6 +61,7 @@ func (g *Group) Wait() error {
 }
 
 // Go calls the given function in a new goroutine.
+// The first call to Go must happen before a Wait.
 // It blocks until the new goroutine can be added without the number of
 // active goroutines in the group exceeding the configured limit.
 //