[S03] Rat**UInt also tries to stay in Rat-space

Make it clear that a Rat raised to a positive integral power also should
try to preserve exactitude as long as Rat can support it.  (Currently
Rat is specced to be Int/uint64, though rakudo does not yet do that.)

S03-operators.pod

@@ -15,8 +15,8 @@ Synopsis 3: Perl 6 Operators
 
     Created: 8 Mar 2004
 
-    Last Modified: 7 Sep 2010
-    Version: 216
+    Last Modified: 11 Sep 2010
+    Version: 217
 
 =head1 Overview
 
@@ -575,10 +575,12 @@ Unless the right argument is a non-negative integer the result is likely to
 be an approximation.  If the right argument is of an integer type,
 exponentiation is at least as accurate as repeated multiplication on
 the left side's type.  (From which it can be deduced that C<Int**UInt>
-is always exact, since C<Int> supports arbitrary precision.)  If the
+is always exact, since C<Int> supports arbitrary precision.
+C<Rat**UInt> is accurate to the precision of C<Rat>.)  If the
 right argument is an integer represented in a non-integer type, the
 accuracy is left to implementation provided by that type; there is
 no requirement to recognize an integer to give it special treatment.
+(Coercion of an integer C<Str> via C<Cool> is likely to do the right thing, however.)
 
 =back