docs: add warning to gpio_get_level()

docs: add warning to gpio_get_level()

On most microcontrollers I have worked with one can retrieve the
current state of output GPIO pins. On ESP32 this is not the case
if the pad is not explictly configured to route this information into
the port by configuring it as input or i/o.
Thus add a warning to the API documentation of gpio_get_level().

Merges https://github.com/espressif/esp-idf/pull/1740