doc/usage.txt improvements
doc/usage.txt improvements

Patch by Greg Sabino Mullane, applied with minor modifications.

kvap/pgbouncer,dbaxa/pgbouncer,wurenny/pgbouncer-x2,wurenny/pgbouncer-x2,jaiminpan/pgbouncer,digoal/pgbouncer-x2,digoal/pgbouncer-x2,dbaxa/pgbouncer,wurenny/pgbouncer-x2,kvap/pgbouncer,jaiminpan/pgbouncer,kvap/pgbouncer,digoal/pgbouncer-x2,dbaxa/pgbouncer,jaiminpan/pgbouncer,wurenny/pgbouncer-x2,digoal/pgbouncer-x2,dbaxa/pgbouncer,kvap/pgbouncer,jaiminpan/pgbouncer