Clarify whether the :@ attribute syntax is required or not

[tmcw]

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This example suggests that you can use a :@ start to dynamic attributes, but doesn't say why, whether it's required or recommended, and whether it changes behavior.

[tmcw]

This is props versus attributes on a closer read - I think I can clarify that.

[Ryuno-Ki]

The prefixes are defined in

https://github.com/11ty/webc/blob/8f10a7c67b6b1eba1dccb66621dc39221f1562a4/src/attributeSerializer.js#L4-L9