Bluetooth: Host: Update doc `user_data` type in `bt_gatt_discover_func_t`

Before this change, the doc on `bt_gatt_discover_func_t` gives the
impression that the type of `user_data` depends on `attr->uuid`. That is
partially incorrect. It is more accurate to say that the type depends on
the discovery type because that determines the ATT operation, which in
turn determines if the data is available.

Add documentation on `user_data` for the discovery type
`BT_GATT_DISCOVER_STD_CHAR_DESC`.

Add a hint `bt_gatt_discover_func_t` that `bt_gatt_read` can do a
different kind of attribute lookup, read-by-type, which may be
more convenient in some cases.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>