Fix deprecation notice for ble::Gap::getState()

The deprecation notice suggests getConnectionCount() which
did not land in the final API. The proper replacement is keeping
your own record and updating during connection and disconnection
callbacks.

Author: LDong-Arm

features/FEATURE_BLE/ble/Gap.h

@@ -1347,13 +1347,15 @@ class LegacyGap :
      * @return The current GAP state of the device.
      *
      * @deprecated Deprecated since addition of extended advertising support.
-     * This is not meaningful when you use extended advertising; please use
-     * isAdvertisingActive() and getConnectionCount().
+     * This is not meaningful when you use extended advertising; Please replace
+     * getState().advertising with isAdvertisingActive(), and replace
+     * getState().connected with your own record and update during callbacks.
      */
     MBED_DEPRECATED_SINCE(
         "mbed-os-5.11.0",
         "Deprecated since addition of extended advertising support. "
-        "Use isAdvertisingActive() and getConnectionCount()."
+        "Replace getState().advertising with isAdvertisingActive()."
+        "Replace getState().connected with your own record and update during callbacks."
     )
     GapState_t getState(void) const;