docs: document <address> elements in one place
docs: document <address> elements in one place

Improve the documentation of what forms a valid <address> element,
since these elements appear in numerous devices.

* docs/formatdomain.html.in (elementsAddress): New section.
(elementsControllers, elementsUSB, elementsNICS, elementsInput)
(elementsHub, elementsCharChannel, elementsSound): Refer to it.

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