net: pkt: Fix net_pkt_pull() documentation

Fix the net_pkt_pull() documentation and mention that the
function can remove data from the start of the network packet.
The function is meant to work similar way net_buf_pull() but
it can handle multiple net_buf fragments.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>