Add description to all types for documentation

Currently the RAPI documentation uses the __str__ of the "types" defined
in ganeti.ht, but some of them are not described. This leads to
descriptions like 'Tuple of ((<Integer <function TInt at
0x2e75578>[...]' (as seen on the official documentation).

Signed-off-by: Aaron Karper <akarper@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>