injection_points: Improve comment about disabled isolation permutation
9f00edc22888 has disabled a permutation due to failures in the CI for
FreeBSD environments, but this is a matter of timing. Let's document
properly why this type of permutation is a bad idea if relying on a wait
done in a SQL function, so as this can be avoided when implementing new
tests (this spec is also a template).
Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/ZyCa2qsopKaw3W3K@paquier.xyz