[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:

• struct definition
• enum definition
• enum value defintion
• UDVTs
• ASTJSON tests for custom errors

[frangio]

Would really like to see this. On top of the ones mentioned in the issue, I'd add custom errors and user defined value types.

It's currently weird because it's possible to add NatSpec in the source code but it doesn't show up in the AST.

[chriseth]

Sounds good!

[PaulRBerg]

An interim solution is to apply a @custom NatSpec tag to document struct fields and enum values. Though I am not sure if I should do it like this:

/// @custom: var Description for my var
struct Foo {
    uint256 var;
}

Or like this:

/// @custom:field var Description for my var
struct Foo {
    uint256 var;
}

What would you recommend, @chriseth? I can't quite tell if there's any meaningful difference between the two, i.e. if one of the two variants produces a more informative ABI.