[AST] Consider expanding the use of NatSpec documentation to all declarations.

[christianparpart]

The motivation behind is, to be able to display additional information about a declaration, not just its type signature.

This might also help documentation.

The relevant AST nodes I'd at least expand to are:

• [x] struct definitions
• [x] enum definitions
• [ ] enum value definitions
• [ ] UDVTs
• [ ] ASTJSON tests for custom errors
• [ ] modules
• [ ] constant variable definitions