Bluetooth: host: Add documentation for OPT_EXT_ADV and bt_le_adv_start

The bt_le_adv_start function can (currently) not be used
together with BT_LE_ADV_OPT_EXT_ADV, and it will
return an -EINVAL error if attempted, but this behavior
is not documented, and several people have asked about
why it does not work.

Signed-off-by: Emil Gydesen <[email protected]>

Author: Thalley

include/zephyr/bluetooth/bluetooth.h

@@ -628,6 +628,8 @@ enum {
 	 *
 	 * @note Enabling this option requires extended advertising support in
 	 *       the peer devices scanning for advertisement packets.
+	 *
+	 * @note This cannot be used with bt_le_adv_start().
 	 */
 	BT_LE_ADV_OPT_EXT_ADV = BIT(10),
 
@@ -1055,6 +1057,9 @@ struct bt_le_per_adv_param {
  * response data parameters are ignored. If the mode is high duty cycle
  * the timeout will be @ref BT_GAP_ADV_HIGH_DUTY_CYCLE_MAX_TIMEOUT.
  *
+ * This function cannot be used with @ref BT_LE_ADV_OPT_EXT_ADV in the @p param.options.
+ * For extended advertising, the bt_le_ext_adv_* functions must be used.
+ *
  * @param param Advertising parameters.
  * @param ad Data to be used in advertisement packets.
  * @param ad_len Number of elements in ad