Solidity function AST nodes get documentation attribute

Solidity AST nodes get the documentation string attribute if the comment above the function matches the NATspec format (3 slashes for now).

I am not really sure I like the fact that having an empty documentation string and having no documentation string can't be differentiated at the moment. Perhaps this should be fixed. Comments are welcome.
What about using an ASTPointer<ASTString>? This has two benefits:
1. It is possible to distinguish between empty documentation and no documentation
2. Consistency with other strings and (you can't know this) more efficient once ASTPointer is converted to plain pointer to fixed memory area and ASTString is converted to range-reference to the source