Add note about access permission checks by inherited TRUNCATE and LOCK TABLE.

Add note about access permission checks by inherited TRUNCATE and LOCK TABLE.

Inherited queries perform access permission checks on the parent
table only. But there are two exceptions to this rule in v12 or before;
TRUNCATE and LOCK TABLE commands through a parent table check
the permissions on not only the parent table but also the children
tables. Previously these exceptions were not documented.

This commit adds the note about these exceptions, into the document.

Back-patch to v9.4. But we don't apply this commit to the master
because commit e6f1e560e4 already got rid of the exception about
inherited TRUNCATE and upcoming commit will do for the exception
about inherited LOCK TABLE.

Author: Amit Langote
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/CA+HiwqHfTnMU6SUkyHxCmpHUKk7ERLHCR3vZVq19ZOQBjPBLmQ@mail.gmail.com