Added a note to the section on 'exec' about the need for a trailing newline

in certain circumstances.  (Apparently, this is a CPython problem.)

Doc/ref/ref6.tex

@@ -612,3 +612,12 @@ for use by \keyword{exec}.
 \bifuncindex{eval}
 \bifuncindex{globals}
 \bifuncindex{locals}
+
+Also, in the current implementation, multi-line compound statements must
+end with a newline:
+\code{exec "for v in seq:\e{}n\e{}tprint v\e{}n"} works, but
+\code{exec "for v in seq:\e{}n\e{}tprint v"} fails with
+\exception{SyntaxError}.
+\exindex{SyntaxError}
+  
+