examples/provisioning: add comment about linker errors
examples/provisioning: add comment about linker errors

If the user copies the BLE example to their own project without understanding the need to enable the BT stack or BTDM BLE settings in the SDK then their build will probably fail at this line due to linker errors.

Closes https://github.com/espressif/esp-idf/pull/6652
Closes IDFGH-4854

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf