Documentation/pktgen: Clearify how-to use pktgen samples

o Change process name in ps output: looks like, these days the process
  is named kpktgend_<cpu>, rather than pktgen/<cpu>.
o Use pg_ctrl for start/stop as it can work well with pgset without
  changes to $(PGDEV) variable.
o Clarify a bit needed $(PGDEV) definition for sample scripts and that
  one needs to `source functions.sh`.
o Document how-to unset a behaviour flag, note about history expansion.
o Fix pgset spi parameter value.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>