Add clarifying comment in HashableClauseMutator

While reading this code last week, it appeared as though there was no
place we ensured that the partition clause actually used equality ops.
As such, I was worried that we might transform a clause such as id < 5
into a constraint like hash(id) = hash(5) when doing shard pruning. The
relevant code seemed to just ensure:

  1. The node is an OpExpr
  2. With a related hash function
  3. It compares the partition column
  4. Against a constant

A superficial reading implied we didn't actually make sure the original
op was equality-related, but it turns out the hash lookup function DOES
ensure that for us. So I added a comment.

src/backend/distributed/planner/multi_physical_planner.c

@@ -2858,6 +2858,13 @@ HashableClauseMutator(Node *originalNode, Var *partitionColumn)
 
 		Oid leftHashFunction = InvalidOid;
 		Oid rightHashFunction = InvalidOid;
+
+		/*
+		 * If operatorExpression->opno is NOT the registered '=' operator for
+		 * any hash opfamilies, then get_op_hash_functions will return false.
+		 * This means this function both ensures a hash function exists for the
+		 * types in question AND filters out any clauses lacking equality ops.
+		 */
 		bool hasHashFunction = get_op_hash_functions(operatorExpression->opno,
 													 &leftHashFunction,
 													 &rightHashFunction);