The name 'probe' (and thus 'probe group') is a relic from the times when
sigrok was mostly about logic analyzers. Nowadays we support a lot more
device types where 'probe' is not really a good term and 'channel' is
much better suited.
This fixes parts of bug #259.
Add @since tags to some (not all yet) public API functions, to document
in which release they were added (and/or in which release there were
API changes).
Document the @since usage in HACKING.
Also, add Doxygen docs for the functions in version.c.