rework PlDoc support using library(pldoc)

[mndrix]

Jan says

Note that your comment_hook will not play nice with PlDoc. An alternative
would be to call doc_collect(true) to make the documentation system collect
comments and use the public interface of doc_modes.pl to get the mode and
type declarations.

I tried this once before but wasn't able to make it work. Try again. I certainly don't want mavis to break all normal PlDoc usage.