testing: define Run result a little more clearly

t.Run(f) does not wait for f after f calls t.Parallel.
Otherwise it would be impossible to create new
parallel sibling subtests for f.

Fixes #22993.

Change-Id: I27e1555ab1ff608eb8155db261d5e7ee8f486aef
Reviewed-on: https://go-review.googlesource.com/83880
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/testing.go

@@ -781,9 +781,9 @@ func tRunner(t *T, fn func(t *T)) {
 	t.finished = true
 }
 
-// Run runs f as a subtest of t called name. It reports whether f succeeded. Run
-// runs f in a separate goroutine and will block until all its parallel subtests
-// have completed.
+// Run runs f as a subtest of t called name. It runs f in a separate goroutine
+// and blocks until f returns or calls t.Parallel to become a parallel test.
+// Run reports whether f succeeded (or at least did not fail before calling t.Parallel).
 //
 // Run may be called simultaneously from multiple goroutines, but all such calls
 // must return before the outer test function for t returns.