Align Access.filter/1 spec with its truthy semantics (#15550)

Author: nseaSeb

lib/elixir/lib/access.ex

@@ -934,7 +934,7 @@ defmodule Access do
 
   """
   @doc since: "1.6.0"
-  @spec filter((term -> boolean)) :: access_fun(data :: list, current_value :: list)
+  @spec filter((term -> as_boolean(term))) :: access_fun(data :: list, current_value :: list)
   def filter(func) when is_function(func) do
     fn op, data, next -> filter(op, data, func, next) end
   end