Remove trailing whitespace and add important note to NSAssert section

Hi, I've read your doc from top to bottom and it's a fantastic resource. The only thing I saw that I would change is to add the important note about not calling functions inside NSAssert because I've seen that happen far too often in production code. I was guilty myself a few times years ago <shameful grin>.

Also included is some harmless trailing whitespace removal that Sublime Text did, if you don't want that, I can put the commit on a separate branch for you to merge without it. --Mark