Update Clownfish documentation

* Parcel prerequisites.
* Single parcel specifier per header file.
* ivars struct.
* Parcel prefix of methods.
* Details of class, function and variable exposure are yet to be
  decided.
* Misc clarifications.
Fix StringIterator documentation
Automate creation of C comments from autogen headers
Don't hardcode POD and man page headers

Use the supplied autogen header and footer instead.