Include documentation in generated RBI file

[dduugg]

Is your feature request related to a problem? Please describe.
I would like signatures and documentation to co-exist, to make browsing a gem easier

Describe the solution you'd like
I would like the documentation included in the generated RBI. (This is a known solution, as Tapioca does this already, browse any of the files here.)

Describe alternatives you've considered

Additional context