Improve docs on custom cache

[strathmeyer]

http://amplifyjs.com/api/request/#custom_cache

Needs:

• information on what the custom cache function should return (scott says, "quick answer: don't return anything if you don't have the value cached. if you do have the value cached, invoke the ajaxSettings.success callback with the value and return false")
• maybe an example?

[jakerella]

Link to the doc section changed: http://amplifyjs.com/api/request/#customcache