Copy section on objectid/isolation restrictions from prop351.

Eventually it belongs in an RPC spec, but for now the relevant discussion goes here.

(See also torspec!292 (merged) where we copy the other parts of prop351 into the relevant part of the spec.)

assigned to @nickm

requested review from @jnewsome

mentioned in issue #1615 (closed)

approved this merge request

mentioned in commit eaef6aa0

merged