Properly implement doc-comment attribute

[ironcev]

Currently, the doc-comment attribute's implementation still relays on a hack that was necessary in the beginning because the attribute arguments didn't support values. Basically, the comment line is stored within an AttributeArg::name.

The knowledge of this workaround has unfortunately spread around the codebase. Different parts of code that analyze doc comments extract the comment from the arg in place and uses different approaches for that (sometimes fetching the first/last argument, sometimes iterating through all arguments, ...). Some of those parts of code will be marked with a TODO linking to this issue.