Added heads up about computed properties to  'marking a model as complete-incomplete' page.

When a person reads this for the first time, he is left wondering why this function calls `property`. Even if there's a resource link in the bottom, it would nice to give a small heads up of whats going on up there.
Merge pull request #852 from steverob/patch-1

Added heads up about computed properties to  'marking a model as complet...