Add C comment about why we don't spell out "month" in interval values.

src/backend/utils/adt/datetime.c

@@ -4049,6 +4049,11 @@ EncodeInterval(struct pg_tm * tm, fsec_t fsec, int style, char *str)
 			/* Compatible with postgresql < 8.4 when DateStyle = 'iso' */
 		case INTSTYLE_POSTGRES:
 			cp = AddPostgresIntPart(cp, year, "year", &is_zero, &is_before);
+			/*
+			 *	Ideally we should spell out "month" like we do for "year"
+			 *	and "day".  However, for backward compatibility, we can't
+			 *	easily fix this.  bjm 2011-05-24
+			 */
 			cp = AddPostgresIntPart(cp, mon, "mon", &is_zero, &is_before);
 			cp = AddPostgresIntPart(cp, mday, "day", &is_zero, &is_before);
 			if (is_zero || hour != 0 || min != 0 || sec != 0 || fsec != 0)