docs: clarify EventuallyWithTf callback vs failure message

The godoc examples implied format strings belong on the condition
callback. Document that msg/args apply only on timeout.

Fixes #1833

Author: leno23

assert/assertion_format.go

@@ -192,8 +192,11 @@ func Eventuallyf(t TestingT, condition func() bool, waitFor time.Duration, tick
 //	}()
 //	assert.EventuallyWithTf(t, func(c *assert.CollectT) {
 //		// add assertions as needed; any assertion failure will fail the current tick
-//		assert.True(c, externalValue, "expected 'externalValue' to be true")
-//	}, 10*time.Second, 1*time.Second, "error message %s", "formatted")
+//		assert.True(c, externalValue)
+//	}, 10*time.Second, 1*time.Second, "externalValue was not true before timeout")
+//
+// The msg and args format the failure when the condition is never met; they are
+// not passed to the condition callback.
 func EventuallyWithTf(t TestingT, condition func(collect *CollectT), waitFor time.Duration, tick time.Duration, msg string, args ...interface{}) bool {
 	if h, ok := t.(tHelper); ok {
 		h.Helper()

assert/assertion_forward.go

@@ -374,8 +374,11 @@ func (a *Assertions) EventuallyWithT(condition func(collect *CollectT), waitFor
 //	}()
 //	a.EventuallyWithTf(func(c *assert.CollectT) {
 //		// add assertions as needed; any assertion failure will fail the current tick
-//		assert.True(c, externalValue, "expected 'externalValue' to be true")
-//	}, 10*time.Second, 1*time.Second, "error message %s", "formatted")
+//		assert.True(c, externalValue)
+//	}, 10*time.Second, 1*time.Second, "externalValue was not true before timeout")
+//
+// The msg and args format the failure when the condition is never met; they are
+// not passed to the condition callback.
 func (a *Assertions) EventuallyWithTf(condition func(collect *CollectT), waitFor time.Duration, tick time.Duration, msg string, args ...interface{}) bool {
 	if h, ok := a.t.(tHelper); ok {
 		h.Helper()

require/require.go

@@ -459,8 +459,11 @@ func EventuallyWithT(t TestingT, condition func(collect *assert.CollectT), waitF
 //	}()
 //	require.EventuallyWithTf(t, func(c *assert.CollectT) {
 //		// add assertions as needed; any assertion failure will fail the current tick
-//		require.True(c, externalValue, "expected 'externalValue' to be true")
-//	}, 10*time.Second, 1*time.Second, "error message %s", "formatted")
+//		require.True(c, externalValue)
+//	}, 10*time.Second, 1*time.Second, "externalValue was not true before timeout")
+//
+// The msg and args format the failure when the condition is never met; they are
+// not passed to the condition callback.
 func EventuallyWithTf(t TestingT, condition func(collect *assert.CollectT), waitFor time.Duration, tick time.Duration, msg string, args ...interface{}) {
 	if h, ok := t.(tHelper); ok {
 		h.Helper()

require/require_forward.go

@@ -375,8 +375,11 @@ func (a *Assertions) EventuallyWithT(condition func(collect *assert.CollectT), w
 //	}()
 //	a.EventuallyWithTf(func(c *assert.CollectT) {
 //		// add assertions as needed; any assertion failure will fail the current tick
-//		assert.True(c, externalValue, "expected 'externalValue' to be true")
-//	}, 10*time.Second, 1*time.Second, "error message %s", "formatted")
+//		assert.True(c, externalValue)
+//	}, 10*time.Second, 1*time.Second, "externalValue was not true before timeout")
+//
+// The msg and args format the failure when the condition is never met; they are
+// not passed to the condition callback.
 func (a *Assertions) EventuallyWithTf(condition func(collect *assert.CollectT), waitFor time.Duration, tick time.Duration, msg string, args ...interface{}) {
 	if h, ok := a.t.(tHelper); ok {
 		h.Helper()