docs(select): Updated ngOptions track by examples

Made the example shown consistent with the advice above it regarding not using
`select as` and `track by` in the same comprehension expression.  Also changed
references to `trackexpr` to `track by` since `trackexpr` is not defined
except in the examples.

Added filter + track by example for ngOptions

The documentation for ngRepeat includes such an example specifying the proper
order for filters and and "track by" clauses in the comprehension expression,
but these docs for ngOptions do not.