dialog: example prompts should follow Material Design guidance

[bisonfoutu]

I just noticed while looking at the overview of the Dialog component that the code sample illustrating dialog structure directives did not follow the Material Design guidelines regarding dialogs and alert dialogs.

For dialogs, avoid ambiguous questions as "Are you sure ?" (even though I'm aware that the dialog's title is "Delete all", but then we could just ask "Delete all elements ?" as a title).
Since the provided dialog surely resembles an alert dialog, the example should also provide some explicit action labels, like "Cancel" and "Delete" instead of "No" and "Yes".

I believe the examples provided in Angular Material's documentation should reflect Material Design guidelines.
I usually expect documentation examples to follow best practices, especially regarding a library as serious as Angular Material.

[Splaktar]

As explained here, this repo is only for reporting issues with the docs infrastructure. Docs content issues should be reported in the angular/components repo, so I'm going to transfer this issue over there.

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