Fix #41 access specifications

Complementary PR for #41.

The specification for accessing properties is written in the docstring for `uca_camera_can_be_written`:
> Check if @prop_name can be written at run-time. This is %FALSE if the
> property is read-only, if uca_camera_set_writable() has not been called or
> uca_camera_set_writable() was called with %FALSE