Fix type descriptions in RAPI documentation

This patch adds descriptors to the “_CheckCIDR*” functions in opcodes
and improves the descriptions generated by “ht.TInstanceOf”, thereby
indirectly fixing bad type descriptions in the RAPI documentation.

Before this patch:
- (String and (<function _CheckCIDRAddrNotation at 0x2f…>))
- (Instance of (<class 'ganeti.opcodes.OpInstanceCreate'>))

After this patch:
- (String and (IPv4 address))
- (Instance of (ganeti.opcodes.OpInstanceCreate))

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>