I haven’t found it documented anywhere. Think of it as an alternative (optional) way to create a new task. As the ordinary way to create a new task below the selected task, by using the return key, doesn’t have the desired effect (it creates a new task inside the action group), it could be worth trying if there was a variant of the command.
Many of the the keyboard shortcuts in Omnifocus can be found in the nicely presented Keyboard Commands page (available in the Help menu). I will let The Omni Group know that option-return is missing there. I will also remind them that I have sent an e-mail to their support a long time ago mentioning that the following convenient shortcuts deserve to be documented there as well as in the Omnifocus manual:
D - Move down
U - Move up
R - Move right
L - Move left
Naturally enough, these commands only work while no text fields are activated.
(There is also a very dangerous command for changing the status to completed: space. A whole set of projects once disappeared when I by mistake tried to write something while having those projects selected. I didn’t realize that this had happened until later, when the projects were missing.)