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

src/include/executor/spi.h

@@ -21,11 +21,14 @@
 
 typedef struct SPITupleTable
 {
+	/* Public members */
+	TupleDesc	tupdesc;		/* tuple descriptor */
+	HeapTuple  *vals;			/* array of tuples */
+
+	/* Private members, not intended for external callers */
 	MemoryContext tuptabcxt;	/* memory context of result table */
 	uint64		alloced;		/* # of alloced vals */
 	uint64		free;			/* # of free vals */
-	TupleDesc	tupdesc;		/* tuple descriptor */
-	HeapTuple  *vals;			/* tuples */
 	slist_node	next;			/* link for internal bookkeeping */
 	SubTransactionId subid;		/* subxact in which tuptable was created */
 } SPITupleTable;