RangeTblEntry.inh vs. RTE_SUBQUERY

Various code comments say that the RangeTblEntry field inh may only be
set for entries of kind RTE_RELATION.

For example

* inh is true for relation references that should be expanded to
include
* inheritance children, if the rel has any. This *must* be false for
* RTEs other than RTE_RELATION entries.

and various comments in other files.

(Confusingly, it is also listed under "Fields valid in all RTEs:", but
that definitely seems wrong.)

I have been deploying some assertions to see if the claims in the
RangeTblEntry comments are all correct, and I tripped over something.

The function pull_up_simple_union_all() in prepjointree.c sets ->inh to
true for RTE_SUBQUERY entries:

/*
* Mark the parent as an append relation.
*/
rte->inh = true;

Whatever this is doing appears to be some undocumented magic. If I
remove the line, then regression tests fail with plan differences, so it
definitely seems to do something.

Is this something we should explain the RangeTblEntry comments?