C code doesn't lend itself well to using classes and pages are best for prose.
A doxygen group is a set of related functions - which is exactly what we have
here, e.g. pw_context.
This patch basically adds the following lines to each header:
\defgroup pw_whatever
\addtogroup pw_whatever
\{
.... function declarations ....
\}
Doxygen is smart enough to merge documentation in the header with
documentation in the correspondin .c file where the function is implemented.