DOC: management: precise some of the fields of "show servers conn"

DOC: management: precise some of the fields of "show servers conn"

As reported in issue #2970, the output of "show servers conn" is not
clear. It was essentially meant as a debugging tool during some changes
to idle connections management, but if some users want to monitor or
graph them, more info is needed. The doc mentions the currently known
list of fields, and reminds that this output is not meant to be stable
over time, but as long as it does not change, it can provide some useful
metrics to some users.