Document that scenario=reminder will be ignored without a toast button

[saschanaz]

See also:

• https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/notifications/win/notification_template_builder.cc;l=122-123;drc=01a77f1d907d16b49466798721936542a0cd26fe
• https://phabricator.services.mozilla.com/D183516

I haven't checked if this also applies to other scenario types, feel free to edit if it does.

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[Jak-MS]

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