Doxygen issues

[beojan]

There are a few places where the Doxygen documentation appears to be processed incorrectly. For example, in Functional, a function called mathtt is listed, which seems to contain the documentation for multiple functions. Similarly, MonadPlus has a mathrm listed.

[ldionne]

Thanks for reporting. Duping to #483.

The fundamental problem is that Doxygen is kind of broken. I just tried to fix some of the issues and failed, I think we may have to rewrite the documentation using a different tool. I remember I had stretched Doxygen to its limit when I wrote it initially, and maybe that wasn't a great idea in retrospect.