testing: document that T and B are safe for concurrent calls

Fixes #13108.

Change-Id: I474cc2a3b7ced1c9eb978fc815f9c6bae9fb3ecc
Reviewed-on: https://go-review.googlesource.com/17235
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/testing.go

@@ -282,6 +282,14 @@ var _ TB = (*B)(nil)
 
 // T is a type passed to Test functions to manage test state and support formatted test logs.
 // Logs are accumulated during execution and dumped to standard error when done.
+//
+// A test ends when its Test function returns or calls any of the methods
+// FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf. Those methods, as well as
+// the Parallel method, must be called only from the goroutine running the
+// Test function.
+//
+// The other reporting methods, such as the variations of Log and Error,
+// may be called simultaneously from multiple goroutines.
 type T struct {
 	common
 	name          string    // Name of test.