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.
For readability yes, but it would make no difference in practice since the skippedComment token in the scanner is kept in a different variable than all the rest of the tokens.