Add docstring support to `define-dbus-object`

[Ambrevar]

Since define-dbus-object defines something akin to methods, it would be nice to attach a documentation string to it. What do you think?

[death]

In principle this makes sense and I would accept a pull request for it, though there are more pressing issues with regards to DEFINE-DBUS-OBJECT and friends (e.g., see https://gist.github.com/death/1c5780c5dcf6a838a867d8701969982f).