testing: document that Log and Logf always print in benchmarks

Fixes #10713.

Change-Id: Ifdafc340ae3bba751236f0482246c568346a569c
Reviewed-on: https://go-review.googlesource.com/9763Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/testing.go

@@ -342,13 +342,15 @@ func (c *common) log(s string) {
 }
 
 // Log formats its arguments using default formatting, analogous to Println,
-// and records the text in the error log. The text will be printed only if
-// the test fails or the -test.v flag is set.
+// and records the text in the error log. For tests, the text will be printed only if
+// the test fails or the -test.v flag is set. For benchmarks, the text is always
+// printed to avoid having performance depend on the value of the -test.v flag.
 func (c *common) Log(args ...interface{}) { c.log(fmt.Sprintln(args...)) }
 
 // Logf formats its arguments according to the format, analogous to Printf,
-// and records the text in the error log. The text will be printed only if
-// the test fails or the -test.v flag is set.
+// and records the text in the error log. For tests, the text will be printed only if
+// the test fails or the -test.v flag is set. For benchmarks, the text is always
+// printed to avoid having performance depend on the value of the -test.v flag.
 func (c *common) Logf(format string, args ...interface{}) { c.log(fmt.Sprintf(format, args...)) }
 
 // Error is equivalent to Log followed by Fail.