Added doxygen comments.

git-svn-id: svn://graphite/diminuto/trunk@597 5dfbf39a-1452-0410-8a97-9e58fedde8e3
Added a log function to throttle.

git-svn-id: svn://graphite/diminuto/trunk@598 5dfbf39a-1452-0410-8a97-9e58fedde8e3
Expanded (by a lot) throttle unit test.
Now also tests variable block sizes, and simulates a long I/O sequence.