docs(common): rewrite docs for NgForOf#ngForTrackBy

[IgorMinar]

Clarify the prupose of the tracking function and document how to create a good one.

Fixes #40461

[IgorMinar]

@TeriGlover can you please take a look at this one as well? thank you!

[mary-poppins]

You can preview f2716b0 at https://pr42329-f2716b0.ngbuilds.io/.

[mary-poppins]

You can preview 4197b82 at https://pr42329-4197b82.ngbuilds.io/.

[atscott]

Hey @IgorMinar, do you think you could take on #35896 (in another PR) as well since it's related? The issue there is that the insertion order of VE just happened to work with ngFor in the select (#35896 (comment)). All the items in the select array are new objects because of the map function so Angular needs to be able to identify which ones are pre-existing via trackBy.

Also, based on that issue maybe it's worth adding a special note in your description to call out the importance of trackBy with select elements and their options.

[IgorMinar]

I'll take a look @atscott. Thanks for the review.

[TeriGlover]

Editing review complete; left several comments.

[TeriGlover]

Editing review completed: #42329 (review). Several comments; otherwise, LGTM.

…

On Tue, May 25, 2021 at 2:26 PM Igor Minar ***@***.***> wrote: I'll take a look @atscott <https://github.com/atscott>. Thanks for the review. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#42329 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AR4DPGWW72XEUIEN5UGAGQDTPQI2HANCNFSM45QF6BVA> .

-- *--Teri Glover--* *Technical Editor*

[TeriGlover]

Hi Igor, What is this second one; where can I find it? Thanks, --Teri

…

On Tue, May 25, 2021 at 1:59 PM Igor Minar ***@***.***> wrote: @TeriGlover <https://github.com/TeriGlover> can you please take a look at this one as well? thank you! — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#42329 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AR4DPGQ5RZCPZ2MP63DSO23TPQFTJANCNFSM45QF6BVA> .

-- *--Teri Glover--* *Technical Editor*

[IgorMinar]

thanks for the review @TeriGlover, I'll work on the additions that Andrew brought up in a new PR and will ask you for a review.

[mary-poppins]

You can preview 4471331 at https://pr42329-4471331.ngbuilds.io/.

[givo]

Hi guys,

This is much better now. Docs were missing the reorder part. Now it'a much more clearer.

Many thanks!

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