Ensure every source file has at least a doxygen @brief description

[rossb]

There is now a "guide to the source code" page in the doxygen documentation: http://www.portaudio.com/docs/v19-doxydocs/srcguide.html

This groups source files into different categories and lists them along with their brief documentation. This is useful (for example) to see a compact list of all test files and what each one does.

The lists are not completely up to date (some files are not listed properly, this needs to be checked and corrected). If you are responsible for a particular file and can't find it in one of the lists it would be a good idea to check that its doxygen comment block is in order.

use @ingroup to specify the group membership of a file. see doc/src/srcguide.dox for the current list of groups a file can belong to

[rossb]

There is now a script at: doc/utils/checkfiledocs.py which soes some simple checks on all source files for minimal doxygen documentation