doc: remove initial * from example code lines

doxygen actually copies that over into the resulting output.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Add more cross-references

doxygen doesn't detect enum cross-references automatically, prefix them with
@ref

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Documentation typo fixes

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Documentation rewordings and additions

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>