cmd/go: fix doc about examples with empty Output

Fixes #18191

Change-Id: Ic2bac9d2a6f42d14e780c74d9c842ee344ab030a
Reviewed-on: https://go-review.googlesource.com/34512Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/alldocs.go

@@ -1606,7 +1606,8 @@
 // is compared exactly against the comment (see examples below). If the last
 // comment begins with "Unordered output:" then the output is compared to the
 // comment, however the order of the lines is ignored. An example with no such
-// comment, or with no text after "Output:" is compiled but not executed.
+// comment is compiled but not executed. An example with no text after
+// "Output:" is compiled, executed, and expected to produce no output.
 //
 // Godoc displays the body of ExampleXXX to demonstrate the use
 // of the function, constant, or variable XXX.  An example of a method M with

src/cmd/go/test.go

@@ -334,7 +334,8 @@ If the last comment in the function starts with "Output:" then the output
 is compared exactly against the comment (see examples below). If the last
 comment begins with "Unordered output:" then the output is compared to the
 comment, however the order of the lines is ignored. An example with no such
-comment, or with no text after "Output:" is compiled but not executed.
+comment is compiled but not executed. An example with no text after
+"Output:" is compiled, executed, and expected to produce no output.
 
 Godoc displays the body of ExampleXXX to demonstrate the use
 of the function, constant, or variable XXX.  An example of a method M with