Fix TLMigrateInstance use of IAllocator

hail now expects correctly that relocate_from is of equal length with
the number of required nodes (fixme: there's a lot of not well
documented behaviour here… not nice for any other potential
IAllocators). As such, we _need_ to pass just the instance's primary
node.

Additionally, update the iallocator doc to correctly specify what this
list (`relocate_from`) contains.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>