doc/eval: substitute if/then/else construct with an example making use of boolean expression composition

Since the if/then/else construct is now supported natively, showing how
to compose such a construct from basic primitives is misleading.

doc/eval.texi

@@ -196,13 +196,13 @@ value, note that:
 
 @code{+} works like OR
 
-and the construct:
+For example the construct:
 @example
-if A then B else C
+if (A AND B) then C
 @end example
-is equivalent to
+is equivalent to:
 @example
-if(A,B) + ifnot(A,C)
+if(A*B, C)
 @end example
 
 In your C code, you can extend the list of unary and binary functions,