[Ada] Adjust List_Length description

[Pierre-Marie de Rodat]

The comment for List_Length says "It is an error to call this function
with No_List (No_List is not considered to be the same as an empty
list).".  It legitimately happens in practice, so adjust the comment to
allow this.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * nlists.ads (List_Length): Adapt comment to match the
        behaviour.
diff --git a/gcc/ada/nlists.ads b/gcc/ada/nlists.ads
--- a/gcc/ada/nlists.ads
+++ b/gcc/ada/nlists.ads
@@ -146,9 +146,9 @@ package Nlists is
    --  No_List. (No_List is not considered to be the same as an empty list).
 
    function List_Length (List : List_Id) return Nat;
-   --  Returns number of items in the given list. It is an error to call
-   --  this function with No_List (No_List is not considered to be the same
-   --  as an empty list).
+   --  Returns number of items in the given list. If called on No_List it
+   --  returns 0, even though No_List is not considered to be the same as an
+   --  empty list.
 
    function Next (Node : Node_Or_Entity_Id) return Node_Or_Entity_Id;
    pragma Inline (Next);