forces/pair_full: support two particle instances

In contrast to pair_trunc, pair_full did not support two different
particle instances. Adding support for this, makes the construction of
the pair forces analogue again.
host/…/pair_trunc: fix bug with Newton's 3rd law

Newton's 3rd law is imposed only if the the two particle instances
agree. Otherwise, the force in particle2 is not updated.

Minor changes for consistency between pair_trunc and pair_full.
mdsim/forces: improve documentation

Add a note on what is actually updated and what triggers the update.
Alert the user that Newton's third law is not obeyed for different
particle instances.