Improve documentation for Quantity

There was a fair bit of information missing on the Quantity docs page, so I've added some here.

I noticed that the docs refer a lot to a method called ``simplify_units()``, but it doesn't exist:

```
In [1]: from astropy import units as u

In [2]: (1. * u.m).simplify_units()
ERROR: AttributeError: 'Quantity' object has no attribute 'simplify_units' [IPython.core.interactiveshell]
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-7afe61635e63> in <module>()
----> 1 (1. * u.m).simplify_units()

AttributeError: 'Quantity' object has no attribute 'simplify_units'
```

So what should the docs be updated to? Or should the code be fixed?

cc @mdboom @adrn @eteq