Update wording in optimizer/README for EquivalenceClasses

d69d45a5a changed how em_is_child members are stored in
EquivalenceClasses.  Children are no longer stored in the ec_members
list.  optimizer/README mentioned that most operations "should ignore
child members", but that felt a little untrue now since child members
are now stored in a separate place, they simply won't be found by the
normal means of looking (a foreach loop over ec_members), and if you don't
find them, there's technically no need to "ignore" them.

Here we tweak the wording slightly to reflect the new storage location
for child members.

Reported-by: Amit Langote <amitlangote09@gmail.com>
Author: Amit Langote <amitlangote09@gmail.com>
Author: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CA+HiwqE8v=EuAP_3F_A2xn8zWx+nG_etW_Fe_DvKO-Fkx=+DdQ@mail.gmail.com

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