Document feature_notifications_panel labs flag

[germain-gg]

For https://github.com/vector-im/element-internal/issues/444

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.

[t3chguy]

Only works with unencrypted content.

This isn't true, it depends on a user's push rules. If a user marked their .m.room.encrypted* push rules with highlight=true then it'd include those too.

[germain-gg]

But in reality, does that ever happen? I'd be more inclined to add smething a bit simpler

[t3chguy]

Its also not right because we don't only show notifications - we show only highlighting notifications. You will have more notifications than you will find in this panel even for unencrypted rooms. notify != highlight

[t3chguy]

In reality if we wish to maintain the highlight behaviour this panel should probably be renamed to Mentions

[germain-gg]

@daniellekirkwood could you help us resolve the wording discussion above?

I'm not convinced we should rebrand this piece of UI at this stage, nor should we go with a too technical description. It might be a good idea to revamp it the day we put some more attention and care to it.

[daniellekirkwood]

Labs flags in product appear to have the structure: Header, subhead so my suggestion would be this:

Enable the notifications panel in the room header
Unreliable in encrypted rooms