Add Doxygen comments to the condvar class. Only the C++ interface
(condvar's methods) is documented, not the alternative C interface
(condvar_* functions).

A reminder: run "doxygen" and point your browser to doxyout/html/index.html
to see the API documentation we have so far.

A lot can still be added to this condvar documentation, including a
good introduction to how to use condition variables, why they have
a mutex, etc. But it's at least a start.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>