libbladeRF: Added sync rx/tx example to doc directory

The existing bladerf_sync_rx() snippet has been replaced by a
more complete snippet in libbladeRF/doc/examples.

From now on, major snippets and examples to be included in Doxygen
documentation shall now be placed in the libbladeRF/doc/examples
directory, and must be buildable and testable.

Use the Doxygen "snippet" command to include the relevant code
from the example program.