Here's the answer to my own question!
Go to the Editor menu on top bar. Select Editor Settings. On the General tab, go to Text Editor -->Completion. Enable "Add Type Hints" checkbox. Mine (Godot v3.2, beta4) was off by default.
Glad you could find it! I've updated the title of your post so people who have the same question can find the answer more easily.
I've been wondering the same thing. Thanks for the answer!