src/README.md: improve docs of core packet ops that clobber their input value
src/README.md: improve docs of core packet ops that clobber their input value

The previously used term “ownership” was confusing because it is only
defined in terms of apps transfering packet ownership via links. This
new phrasing states that the affected operations “free” their packet
argument which should be intuitively understandable by most programmers,
especially given the explanation of packet.free.

SnabbCo/snabbswitch,alexandergall/snabbswitch,Igalia/snabb,dpino/snabbswitch,eugeneia/snabb,snabbco/snabb,dpino/snabbswitch,eugeneia/snabb,dpino/snabb,Igalia/snabbswitch,eugeneia/snabbswitch,eugeneia/snabb,snabbco/snabb,snabbco/snabb,eugeneia/snabbswitch,eugeneia/snabb,Igalia/snabb,alexandergall/snabbswitch,Igalia/snabb,eugeneia/snabbswitch,SnabbCo/snabbswitch,Igalia/snabb,dpino/snabbswitch,dpino/snabb,eugeneia/snabb,dpino/snabb,alexandergall/snabbswitch,Igalia/snabbswitch,SnabbCo/snabbswitch,alexandergall/snabbswitch,eugeneia/snabb,SnabbCo/snabbswitch,snabbco/snabb,dpino/snabb,Igalia/snabbswitch,eugeneia/snabb,alexandergall/snabbswitch,snabbco/snabb,Igalia/snabb,Igalia/snabbswitch,snabbco/snabb,dpino/snabbswitch,Igalia/snabb,snabbco/snabb,Igalia/snabb,Igalia/snabb,dpino/snabb,dpino/snabb,alexandergall/snabbswitch,alexandergall/snabbswitch,Igalia/snabbswitch,dpino/snabb,eugeneia/snabbswitch,alexandergall/snabbswitch,eugeneia/snabb,snabbco/snabb