clearly state responsibilities when using is pure

Author: gfldex

doc/Type/Routine.pod6

@@ -134,8 +134,8 @@ Calculating 43th prime
 Marks a subroutine as I<pure>, that is, it asserts that for the same input, it
 will always produce the same output without any additional side effects.
 
-This is a hint to the compiler that it can constant-fold calls to such
-functions when the arguments are known at compile time.
+This is a promise by the programmer to the compiler that it can constant-fold
+calls to such functions when the arguments are known at compile time.
 
     sub double(Numeric:D $x) is pure {
         2 * $x;