rework DESCRIPTION for both dhcpleased(8) and slaacd(8) to more accurately

rework DESCRIPTION for both dhcpleased(8) and slaacd(8) to more accurately
reflect how they work, to emphasize that they do nothing unless auto conf
is set, and to more closely match each other;

prompted by a diff from andrew christopher hawk on tech, who noted a wonky
sentence;

help/feedback deraadt florian; ok deraadt