[DOC beta] Cleanup Ember.TextField docs.

Before: 

![screenshot](https://www.monosnap.com/image/vIcBQxbMhxy6LUtkdkacrmLqB.png)

After:

![screenshot](https://www.monosnap.com/image/y9aw41URJs20I1gIRwc2STZV4.png)