Clarify the distinction between public and private SPITupleTable fields.

Clarify the distinction between public and private SPITupleTable fields.

The fields that we consider public are "tupdesc" and "vals", which
historically are in the middle of the struct.  Move them to the front
(this should be perfectly safe to do in HEAD) and add comments to make
it quite clear which fields are public or not.

Also adjust spi.sgml's documentation of the struct to match.
That doc had bit-rotted somewhat, as it was missing some fields.
(Arguably we should just remove all the private fields from the docs,
but for now I refrained.)

Daniel Gustafsson, reviewed by Fabien Coelho

Discussion: https://postgr.es/m/0D19F836-B743-4340-B6A2-F148CA3DD1F0@yesql.se