SPITupleTable members missing in docs

Commit 3d13623d75d3206c8f009353415043a191ebab39 added the next and subid fields
to the SPITupleTable struct, but they never made it into the documentation.
While these are internal members, we already document several other internal
ones (with a sentence on not using them) so add these too to make the
documentation match reality.

Since this makes the number of internal members far outnumber the public ones,
also reword the statement about which fields can be used to try and improve
clarity.

cheers ./daniel