Add documentation on using Flash alerts in dialog components

[emplums]

I came across this usage of a Flash alert inside of a details-dialog that felt a bit off:

Should we add some documentation about whether or not Flash alerts should be used inside of dialog components? To me it feels a bit redundant since the dialog itself is sort of like an alert but I wasn't 100% sure if this is something we should discourage or not.

[simurai]

Can't find the issue/PR, but there was a case recently where a user could do different actions inside the modal and it needed some sort of waning/error message. So maybe in that case it's fine to use a flash alert? The guide could be:

Only use a flash alert in modals in response to an action taken.

[yaili]

This also seems related to danger patterns, maybe we could document a good example of a warning before doing something destructive?