testing: mention in docs for Logf that a final newline is added if needed

Fixes #16423

Change-Id: I9635db295be4d356d427adadd309084e16c4582f
Reviewed-on: https://go-review.googlesource.com/31255Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/testing.go

@@ -476,10 +476,11 @@ func (c *common) log(s string) {
 // 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. 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.
+// Logf formats its arguments according to the format, analogous to Printf, and
+// records the text in the error log. A final newline is added if not provided. 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.