Document the schema so that everyone knows the purpose and scope of your schema.

Document all the global components of your schema.

Document your includes, imports, and redefines so the next schema author knows why they are there.

Document local type definitions as they aren't going to be documented elsewhere.

Any enumeration or options should be document so a user can choose between them.

When there is any doubt, add some kind of documentation.

Document your assumptions. Keep in mind that the schema definitions and declarations are not enough. You might think a name is obvious but they might have different definitions in different contexts.