All this is relevant when you have invented some XML structure that will be used by others and you want to make sure they'll understand it.

When you're serious about documenting, invest in a tool-chain that automates at least the generation of element/attribute documentation.

Acknowledge you're in a knowledge bubble. Try to distance yourself from what you know before you begin.

Always add a schema of some sort. But that's not enough.

Take the narrative as a starting point and include generated parts into your (hand-crafted) work. Generated documentation is almost never sufficient, satisfactory and/or clear enough.

Start by taking the reader by the hand in some overview introduction. Provide one or more examples.

Spend some time making it look nice.