testing: emphasize that Fatal/FailNow/etc run deferred calls

Fixes #22989.

Change-Id: I9776a7b0d0598c2cb118c323d1f8f933665da254
Reviewed-on: https://go-review.googlesource.com/83881
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/testing/testing.go

@@ -520,7 +520,8 @@ func (c *common) Failed() bool {
 }
 
 // FailNow marks the function as having failed and stops its execution
-// by calling runtime.Goexit.
+// by calling runtime.Goexit (which then runs all deferred calls in the
+// current goroutine).
 // Execution will continue at the next test or benchmark.
 // FailNow must be called from the goroutine running the
 // test or benchmark function, not from other goroutines