Update IAllocator interface documentation

Update the missing documentation for the tags fields of node groups.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
(partially cherry picked from commit 2daca99b)

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>