Updated ramdisk API docstrings
Updated ramdisk API docstrings

Added information about raised exceptions and conditions
under which they are raised.

Change-Id: Idde5f4da91cee51603a27720efbdc18601f9b780

pshchelo/ironic,SauloAislan/ironic,SauloAislan/ironic,NaohiroTamura/ironic,pshchelo/ironic,NaohiroTamura/ironic,openstack/ironic,openstack/ironic