testing: clarify when Cleanup is called

Change-Id: I2ed394fccec83aac09a49673554cbf504787965b
Reviewed-on: https://go-review.googlesource.com/c/go/+/306810
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

src/testing/testing.go

@@ -876,7 +876,7 @@ func (c *common) Helper() {
 	}
 }
 
-// Cleanup registers a function to be called when the test and all its
+// Cleanup registers a function to be called when the test (or subtest) and all its
 // subtests complete. Cleanup functions will be called in last added,
 // first called order.
 func (c *common) Cleanup(f func()) {