fix(battery): gate Full-as-Plugged behind full-at-plugged option, drop debug output

Reporting a full battery on AC as 'Plugged' is now opt-in via the
'full-at-plugged' config option (default false), so existing format-full /
tooltip-format-full setups keep working. Also documents the option.

src/modules/battery.cpp

@@ -706,7 +706,7 @@ auto waybar::modules::Battery::update() -> void {
   if (!adapter_.empty()) {
     std::ifstream(adapter_ / "online") >> adapter_online;
   }
-  if (status == "Full" && adapter_online) {
+  if (config_["full-at-plugged"].asBool() && status == "Full" && adapter_online) {
     status = "Plugged";
   }
   auto status_pretty = status;