MINOR: stats: improve pending connections description

In order to unify prometheus and stats description, we need to clarify
the description for pending connections.
- remove the BE reference in counters struct, as it is also used in
  servers
- remove reference of `qcur` field in description as it is specific to
  stats implemention
- try to reword cur and max pending connections description

Signed-off-by: William Dauchy <wdauchy@gmail.com>
2 files changed