Add explanation for the depth on updated projection

[pwbh]

Thought it would also be a good idea to add an explanation on why depth it calculated like it is, feel free to tweak it to how you see fit Gregg.

[pwbh]

@greggman care to take a look? :)

[greggman]

Thank you for the pull request. It's a good idea to explain this a little more I think. I'm not sure what the best wording is yet though.