Documentation for by.repeater

[keithrz]

1. Please mention in the documentation that the "track by" section of a repeater is not needed. It was mentioned in this SO discussion:
   http://stackoverflow.com/questions/37517930/how-to-select-elements-using-protractor-created-by-ng-repeat-track-by-index
2. Examples with nested repeaters would be very helpful.

Thanks!

[sjelin]

Anyone want to write this? Just add it to the comment here

[Jayasankar-m]

Added a pull request #4983 as per comments