Adjust explanation of operator precedence summary -- exponentiation groups

from right to left!

Closes bug #125391.

Doc/ref/ref5.tex

@@ -899,7 +899,8 @@ precedence (least binding) to highest precedence (most binding).
 Operators in the same box have the same precedence.  Unless the syntax
 is explicitly given, operators are binary.  Operators in the same box
 group left to right (except for comparisons, which chain from left to
-right --- see above).
+right --- see above, and exponentiation, which groups from right to
+left).
 
 \begin{tableii}{c|l}{textrm}{Operator}{Description}
     \lineii{\keyword{lambda}}			{Lambda expression}