[DOC beta] Cleanup Ember.TrackedArray docs.

Before:

![before
screenshot](https://www.monosnap.com/image/8nKA4IADSYDrGOBuc3vvDLrKe.png)

After:

![after
screenshot](https://www.monosnap.com/image/nOvjUhPLt98DB7DBjN62iwgio.png)

Before:

![before
screenshot](https://www.monosnap.com/image/rGArTSGTO5x1zSZc5ZWpH3ndr.png)

After:

![after
screenshot](https://www.monosnap.com/image/CBRu8oWAuw1jsEPtK739vkl3V.png)