fix arity requirements for function combinator

Author: gfldex

doc/Type/Callable.pod6

@@ -43,7 +43,10 @@ corresponding parameters.
 =head2 C«infix:<∘>»
 X<|∘,function combinator>X<|o,function combinator>
 
-The function combinator C«infix:<∘>» or C«infix:<o>» combines two functions of arity one, so that the left function is called with the return value of the right function.
+The function combinator C«infix:<∘>» or C«infix:<o>» combines two functions, so
+that the left function is called with the return value of the right function.
+The number of return values of the right function has to match the arity of the
+left function.
 
     sub f($p){ say 'f'; $p / 2 }
     sub g($p){ say 'g'; $p * 2 }