fix invalid decumentation for using Ember.compute.empty with arrays
Merge pull request #3579 from stefanpenner/fix-empty-cp-docs

fix invalid decumentation for using Ember.compute.empty with arrays
[fixes #3072] Ember.run.later now behaves closer to setTimeout +
Ember.run
Ensure Ember.run.laters executeAt date is correclty coerced.