[DOC beta] Update Object Root docs re: plurals

[elithrar]

This partially addresses some of the documentation requirements in emberjs/ember.js#12319

Specifically:

• Clarification that root keys can be pluralized regardless—Ember not being strict about this may help others understand whether they need to normalize a response.
• A further note that—if a HTTP server responds to a findRecord request for a specific ID (or other param) with more than one object in the array—Ember Data will only parse the object with the matching ID/param.

This is low hanging fruit but I wanted to test the appetite for small doc changes like this—I suspect I'll have a few as I work through some Go + Ember stuff!

[wecc]

There are a couple of code style errors (trailing whitespace) causing Travis to fail. Could you correct them?

[elithrar]

@wecc - done. Last time I use the GitHub markdown editor for a quick fix ;)

[wecc]

Thanks @elithrar!