Clarify working in 'top x items using sequential scan' item

doc/TODO

 TODO list for PostgreSQL
 ========================
-Last updated:		Mon Feb 11 13:14:25 EST 2002
+Last updated:		Tue Feb 12 17:24:00 EST 2002
 
 Current maintainer:	Bruce Momjian (pgman@candle.pha.pa.us)
 
@@ -409,6 +409,7 @@ OPTIMIZER / EXECUTOR
 * Make IN/NOT IN have similar performance to EXISTS/NOT EXISTS [exists]
 * Missing optimizer selectivities for date, r-tree, etc. [optimizer]
 * Allow ORDER BY ... LIMIT to select top values without sort or index
+  using a sequential scan for highest/lowest values
 
 
 MISCELLANEOUS