docs(select): Clarify documentation 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 documents this.

Closes angular/angular.js#6564

Part 1 of the split of angular/angular.js#8544 as requested by @btford