I see that it says use legacy my-hacks.php. I'm guessing this may be phased out in some future release and/or it is not the best way to do things.
I have some custom functions that I have in my-hacks.php that just do some simple things - most just return some code.
What is the correct way to implement small little functions I write (i.e. to return some navigation code, where I just say get_navigation();) so that I don't have to worry about updating in the future?