flatmap is the last element of Any that needed rewriting

So this closes #1731

Author: JJ

doc/Type/Any.pod6

@@ -602,9 +602,20 @@ Defined as:
 
     method flatmap(&block, :$label)
 
-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>.
+B<DEPRECATION NOTICE>: This method is deprecated in 6.d and will be removed in
+6.e. Use C<.map> followed by C<.flat> instead.
+
+Applies L<C<map>|/type/Any#method_map> to every element with the block and
+C<Label> used as an argument and flattens out the result using C<.flat>.
+
+    say "aabbccc".comb.Mix.flatmap: "→ " ~ *; # OUTPUT: «(→ b␉2 → c␉3 → a␉2)␤»
+
+In this case, the elements of the C<Mix> are itemized to key␉value, and then
+mapped and flattened. Same result as
+
+    say "aabbccc".comb.Mix.map( "→ " ~ * ).flat
+
+Which is why it is deprecated in 6.d and will be eventually eliminated in 6.e.
 
 =head2 method roll