ethdev: improve API comments of close and detach functions

The patch fixes doxygen comments of below functions.
 - rte_eth_dev_close()
   Add description about when the function can be called and also about
   what kind of resources will be freed.
 - rte_eth_dev_detach()
   Add description about when the function can be called.

Signed-off-by: Tetsuya Mukawa <mukawa@igel.co.jp>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>