testing: document whitespace trimming of example expected/actual output.

Fixes #4642.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/7090044

src/pkg/testing/testing.go

@@ -42,8 +42,8 @@
 //
 // The package also runs and verifies example code. Example functions may
 // include a concluding comment that begins with "Output:" and is compared with
-// the standard output of the function when the tests are run, as in these
-// examples of an example:
+// the standard output of the function when the tests are run. (The comparison
+// ignores leading and trailing space.) These are examples of an example:
 //
 //     func ExampleHello() {
 //             fmt.Println("hello")