[DOC] Clarify wording for Ember.makeArray

Clarified the documentation for Ember.makeArray to follow the "if...it will" convention found in other parts of the documentation. 

BEFORE

  Forces the passed object to be part of an array. If the object is already
  an array or array-like, returns the object. Otherwise adds the object to
  an array. If obj is `null` or `undefined`, returns an empty array.


AFTER

  Forces the passed object to be part of an array. If the object is already
  an array or array-like, it will return the object. Otherwise, it will add the object to
  an array. If obj is `null` or `undefined`, it will return an empty array.