docs(ngFor): add documentation for ngForTrackBy

[lacolaco]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

No documentation exists for ngForTrackBy

What is the new behavior?

Added

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

In the plunker demo (http://plnkr.co/edit/KVuXxDp0qinGDyo307QW?p=preview), ngForTrackBy is not used. I think it also should be updated.

[vicb]

Use trackBy, wo prefix

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