Added description of DISTINCT/DISTINCTROW/ALL options in SELECT.

Docs/manual.texi

@@ -33100,6 +33100,13 @@ mysql> select user,max(salary) AS sum from users
     ->        group by user HAVING sum>10;
 @end example
 
+@item
+The options @code{DISTINCT}, @code{DISTINCTROW} and @code{ALL} specify
+whether duplicate rows should be returned. The default is (@code{ALL}),
+all matching rows are returned. @code{DISTINCT} and @code{DISTINCTROW}
+are synonyms and specify that duplicate rows in the result set should
+be removed.
+
 @item
 All options beginning with @code{SQL_}, @code{STRAIGHT_JOIN}, and
 @code{HIGH_PRIORITY} are MySQL extensions to ANSI SQL.