Added documentation for preload parameter

Documented the purpose of the `preload` parameter on the `find` and `findById` functions
Added an example of how to use the preload arg

As per @bmac's suggestion to swipe the example from system/model.js's _preloadData function
Fixed typo

Fixed typo caught by @chrisvariety
Removed a duplicate explanation of preload

Removed what is essentially a duplicate explanation of `preload` as per @igorT
Merge pull request #2198 from ctataryn/patch-1

Added documentation for preload parameter