Bluetooth: host: clarify which GATT subscribe flags are internal

Add a note to the GATT subscription flags' docstring to indicate what flags
are used internally and are not part of the API.

We might want to move them in their own struct member, something like
`internal_flags` in a subsequent change.

Signed-off-by: Jonathan Rico <[email protected]>

Author: jori-nordic

include/zephyr/bluetooth/gatt.h

@@ -1778,6 +1778,8 @@ enum {
 	 *
 	 *  If set, indicates write operation is pending waiting remote end to
 	 *  respond.
+	 *
+	 *  @note Internal use only.
 	 */
 	BT_GATT_SUBSCRIBE_FLAG_WRITE_PENDING,
 
@@ -1788,6 +1790,8 @@ enum {
 	 *
 	 *  Used to avoid sending subscription requests multiple times when the
 	 *  @kconfig{CONFIG_BT_GATT_AUTO_RESUBSCRIBE} quirk is enabled.
+	 *
+	 *  @note Internal use only.
 	 */
 	BT_GATT_SUBSCRIBE_FLAG_SENT,