doc: update callback functions in pipeline sample guide
doc: update callback functions in pipeline sample guide

Update f_post_init for pipeline frontend.
Move f_track from pipeline backend to pipeline frontend.

Signed-off-by: Bao-Long Tran <dd4b43c1703aa83532bb2c360f81427f3c5605a9@viettel.com.vn>
Acked-by: Marko Kovacevic <32a723043e16bf2a3185e32c2e613ae1ee55288f@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk