Clarify documentation on Finish

Finish no longer has to be explicitly called on Controller with defer
if the Controller was created with testing.T.

We tried to just mark this as deprecated in uber-go#50 but doing so would break
many users, so we reverted it in uber-go#85.

In doing so we dropped some wordings from the doc that marked Finished
as unnecessary for most cases.

This PR fixes the docstring to guide users without explicitly marking
the API deprecated.

gomock/controller.go

@@ -48,8 +48,7 @@ type cleanuper interface {
 // A Controller represents the top-level control of a mock ecosystem.  It
 // defines the scope and lifetime of mock objects, as well as their
 // expectations.  It is safe to call Controller's methods from multiple
-// goroutines. Each test should create a new Controller and invoke Finish via
-// defer.
+// goroutines. Each test should create a new Controller.
 //
 //	func TestFoo(t *testing.T) {
 //	  ctrl := gomock.NewController(t)
@@ -240,6 +239,9 @@ func (ctrl *Controller) Call(receiver any, method string, args ...any) []any {
 
 // Finish checks to see if all the methods that were expected to be called were called.
 // It is not idempotent and therefore can only be invoked once.
+//
+// Note: If you pass a *testing.T into [NewController], you no longer
+// need to call ctrl.Finish() in your test methods.
 func (ctrl *Controller) Finish() {
 	// If we're currently panicking, probably because this is a deferred call.
 	// This must be recovered in the deferred function.