net: adjust rejected connection metric help text

Adjust the existing metric text to clarify that is connections
rejected for hitting the connection count cap, not the connection
rate limit cap (as we are shortly introducing a metric for the
latter).

Ref CORE-6827.

src/v/net/probes.cc

@@ -60,7 +60,8 @@ void server_probe::setup_metrics(
  "connections_rejected",
  [this] { return _connections_rejected; },
  sm::description(ssx::sformat(
- "{}: Number of connections rejected for hitting connection limits",
+ "{}: Number of connection attempts rejected for hitting open "
+ "connection count limits",
  proto))),
  sm::make_counter(
  "requests_completed",