Modified keyed update on ToC docs

[phamduylong]

Linked Issue

Closes #2294

Description

Added a bit more descriptive information on which typical case would be useful for keyed update, made changes to code block.

Couldn't find a better way to animate this and give a better example. Do feedback if you have better ideas in mind.

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[endigo9740]

@phamduylong We tend to favor ultra minimal examples, otherwise folks are prone to check out, skim, and miss valuable information. Here's what I've worked up:

[phamduylong]

@endigo9740 I think part of the original issue was that tabIndex wasn't supposed to be const (since it was intended to change). Otherwise looks good to me.

[endigo9740]

@phamduylong I was pushing a fix for that when you responded! Screenshot updated above.