Document consequences of calling t.FailNow()

These comments are adapted from t.FailNow()'s own documentation.

Closes #1701

mock/mock.go

@@ -332,6 +332,8 @@ func (m *Mock) TestData() objx.Map {
 */
 
 // Test sets the test struct variable of the mock object
+// Test should not be called on an object that is going to be used in a
+// goroutine other than the one running the test function.
 func (m *Mock) Test(t TestingT) {
 	m.mutex.Lock()
 	defer m.mutex.Unlock()

require/doc.go

@@ -23,6 +23,8 @@
 //
 // The `require` package have same global functions as in the `assert` package,
 // but instead of returning a boolean result they call `t.FailNow()`.
+// A consequence of this is that it must be called from the goroutine running
+// the test function, not from other goroutines created during the test.
 //
 // Every assertion function also takes an optional string message as the final argument,
 // allowing custom error messages to be appended to the message the assertion method outputs.