Adds deprecation notice

Which is taken from [here](https://github.com/perl6/roast/blob/master/docs/announce/6.d.md)

Author: JJ

doc/Type/Any.pod6

@@ -600,13 +600,11 @@ don't really have keys.
 
 Defined as:
 
-    method flatmap(Any:U: &code --> Seq)
+    method flatmap(&block, :$label)
 
-Coerces the invocant to a C<list> by applying its
-L«C<.list>|/routine/list» method and uses
-L«C<List.flatmap>|/type/List#method_flatmap» on it.
-
-    say Any.flatmap({.reverse}); # OUTPUT: «((Any))␤»
+N<This method is deprecated in 6.d and will be removed in 6.e>
+Applies L<C<map>|/type/Any#method_map> to every element with the block and label
+used as an argument and flattens out the result using C<.flat>.
 
 =head2 method roll