@-tags include everything after them

[lpw25]

I'm pretty sure I implemented this behaviour once upon a time and I did so to match ocamldoc's existing behaviour. But I wonder if it is a mistake to have @tags extend onwards until either the end of the comment or another @tag. I just found something like:

(** Some docs about things

     @param foo Stuff about a parameter
     
     More stuff about the whole function, not just the parameter. *)

and it seems a very reasonable thing to write, but it gets rendered very badly because everything after @param is considered part of the tag. (Also #329 is right, the markup for tags is bad).

Perhaps we should take a blank line as ending the tag? I think the actual tags we support would be fine only taking a single paragraph as input.