On partition detach, we acquire a SHARE lock on all tables that
reference the partitioned table that we're detaching a partition from,
but failed to document this fact.  My oversight in commit f56f8f8d.
Repair.  Backpatch to 12.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20210325180244.GA12738@alvherre.pgsql