Full doxygen for every API function

The code comments are fairly poor in this module. None of the API functions have doxygen @param declarations, and only one has @return. There are some functions where doxygen is missing entirely (eg. twitter_account_load()). These comments are really important if you want others to review and help debug your code.

Things like form functions and hook implementations don't need much detail, but they do need some basic description (most have that already). Class methods also need full doxygen.