Issue with priority of execution in filters

[puneeth8994]

Need to use track by $index at the end in ng-repeat or else the filter considers $index as a value that should be filtered out. This seems to be common issue.

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
docs update

What is the current behavior? (You can also link to an open issue here)
priority of execution is important while using filters, limit, and track by $index

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change?

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

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[puneeth8994]

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[Narretz]

In general, the docs say that the track by expression must always go at the end, so it's definitely not optional to do it this way.

However, you get a different error message when you include the "as" part in the filter: https://docs.angularjs.org/error/$parse/syntax?p0=as&p1=is an unexpected token&p2=20&p3=$index | filter:q as results&p4=as results, see https://plnkr.co/edit/4tSsoInjMZkLJs0AUmtC?p=preview

That means if we put an explanation in the notarray error message, we are missing errors that trigger the parse syntax error.
Ideally, we would add this info to https://code.angularjs.org/snapshot/docs/error/ngRepeat/iexp, but since this is triggered before Angular parses the expression, this is difficult to do.

[Narretz]

I don't feel like this makes the docs better, for the reasons pointed out above. I've opened a new issue to track a possible improvement to the ngRepeat expression parsing: #16023