doc/org.texi: tell where Org put SCHEDULED and DEADLINE.

Org puts this information on the line right after the headline.
If the user adds information between this line and the headline,
especially timestamps, then Org might be confused.  Ask the user
not to put information there.

doc/org.texi

@@ -5575,7 +5575,9 @@ sexp entry matches.
 @node Inserting deadline/schedule, Repeated tasks, Deadlines and scheduling, Deadlines and scheduling
 @subsection Inserting deadlines or schedules
 
-The following commands allow you to quickly insert a deadline or to schedule
+The following commands allow you to quickly insert@footnote{The @samp{SCHEDULED} and
+@samp{DEADLINE} dates are inserted on the line right below the headline.  Don't put
+any text between this line and the headline.} a deadline or to schedule
 an item:
 
 @table @kbd