Fixes the last "return Any"

By looking at the actual implementation. Revised also other "returns empty list". It actually returns an empty list in many cases, so it's OK. I haven't seen any other remainint, so provisionally closes #1731. If anyone finds a particular description that's wrong, I would rather have an specific open issue for that.

Author: JJ

doc/Type/Any.pod6

@@ -553,7 +553,7 @@ Defined As:
     multi method keys(Any:U: --> List)
     multi method keys(Any:D: --> List)
 
-For defined L<Any> returns its L<keys>, otherwise returns an empty list.
+For defined L<Any> returns its L<keys> after calling `list` on it, otherwise calls `list` and returns it.
 
     say Any.keys; # OUTPUT: «()␤»
 
@@ -874,7 +874,7 @@ Defined As:
 
     method tree(--> Any)
 
-Returns Any.
+Returns the class if it's undefined or if it's not iterable, returns the result of applying the `tree` method to the elements if it's `Iterable`.
 
     say Any.tree; # OUTPUT: «Any␤»