man: clarification when both cntr and CQ attached

When both counter and CQ are attached to the endpoint it
makes sense to just update the counter on success and
generate completion entries for errors.

Signed-off-by: Sayantan Sur <sayantan.sur@intel.com>
Merge pull request #414 from sayantansur/pr-cntr-cq

man: clarification when both cntr and CQ attached