Fix description and grouping of RangeTblEntry.inh

Fix description and grouping of RangeTblEntry.inh

The inh field of RangeTblEntry was doubly confusingly documented.
Some parts of the code insisted that it was only valid for
RTE_RELATION entries, other parts said the field was valid for all
entries.  Neither was quite correct.  More correctly, the field is
valid for RTE_RELATION entries but is also used in the planner for
RTE_SUBQUERY entries.  So it makes more sense to group it with other
fields that are primarily for RTE_RELATION but borrowed by
RTE_SUBQUERY.  (The exact position was chosen so that it is next to
relkind for better struct packing, and next to relid, since relid and
inh are sort of the input fields and the others are filled in later.)
Also add documentation for the planner's use at the struct definition.

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