Fix thinko in comment

The listed numbers disagreed with the ones being used in the symbols;
but instead of just fixing the numbers in the comment, use the symbolic
name instead, which seems clearer.

This has been wrong all along, so apply back to 9.5 where BRIN was
introduced.

Reported-by: Tomas Vondra
Discussion: https://postgr.es/m/5ff514f2-8b1e-6366-b11c-8e2ed442562d@2ndquadrant.com

src/backend/access/brin/brin_inclusion.c

@@ -59,10 +59,14 @@
 /*-
  * The values stored in the bv_values arrays correspond to:
  *
- * 0 - the union of the values in the block range
- * 1 - whether an empty value is present in any tuple in the block range
- * 2 - whether the values in the block range cannot be merged (e.g. an IPv6
- *	   address amidst IPv4 addresses).
+ * INCLUSION_UNION
+ *		the union of the values in the block range
+ * INCLUSION_UNMERGEABLE
+ *		whether the values in the block range cannot be merged
+ *		(e.g. an IPv6 address amidst IPv4 addresses)
+ * INCLUSION_CONTAINS_EMPTY
+ *		whether an empty value is present in any tuple
+ *		in the block range
  */
 #define INCLUSION_UNION				0
 #define INCLUSION_UNMERGEABLE		1