Monodevelop automatically generates verbose documentation for functions and classes if "///" is typed in the appropriate place, upon typing the third '/'.
I want it to go over all of my code, though - I thought there was some button somewhere, and I looked around in the drop-down menus, tried right-clicking file names, etc. I could not find such an option - does it not exist? I could not find a plugin that does that either.
It is not possible. Your options are:
Online searches have not come up with any plugins for monodevelop that would do that, and the option does not exist in the vanilla IDE.