fix(select): Clarify documentation and add exception for ngOptions

ngOptions introduced `track by` in c32a859bdb93699cc080f9affed4bcff63005a64.
Using `track by` puts constraints on the value you can use in the interpolation
expression in ngOptions. This patch both documents this and adds an exception
if you use ngOptions in an unsupported way.

Closes angular/angular.js#6564