Improve field order in RangeTblEntry

Improve field order in RangeTblEntry

When perminfoindex was added, it was just added at the end of the
block.  It would make sense to keep it closer to more related fields.
In passing, also add an inline comment, like the other fields have.
(Other field reorderings and documentation improvements in
RangeTblEntry are being discussed, but it's better not to mix them
together.)

Discussion: https://www.postgresql.org/message-id/flat/6c1fbccc-85c8-40d3-b08b-4f47f2093711%40eisentraut.org