Add comment that attributes.is_nullable was removed from SQL standard

We don't have to remove the column if no one is bothered, but it's
useful to comment on it in case someone looks for it in newer
standards versions.

src/backend/catalog/information_schema.sql

@@ -274,7 +274,7 @@ CREATE VIEW attributes AS
            CAST(pg_get_expr(ad.adbin, ad.adrelid) AS character_data) AS attribute_default,
            CAST(CASE WHEN a.attnotnull OR (t.typtype = 'd' AND t.typnotnull) THEN 'NO' ELSE 'YES' END
              AS yes_or_no)
-             AS is_nullable,
+             AS is_nullable, -- This column was apparently removed between SQL:2003 and SQL:2008.
 
            CAST(
              CASE WHEN t.typelem <> 0 AND t.typlen = -1 THEN 'ARRAY'