Document Any.append.

As of moar source code 2017-12.
See issue #1731

Author: fluca1978

doc/Type/Any.pod6

@@ -916,10 +916,14 @@ Based on C<$matcher> value can be either C<((Any))> or empty List.
 
 Defined As:
 
-    method append(@values --> Array)
-
-Initializes Any variable as empty Array and calls
-L«C<Array.append>|/type/Array#method_append» on it.
+    proto method append(|) is nodal {*}
+    multi method append(Any:U \SELF: |values --> Array)
+
+In the case the instance is not a positional-thing, it instantiate it
+as a new L<Array|/type/Array>, otherwise clone the current
+instance.
+After that, it appends the values passed as arguments to
+the array obtained calling L«C<Array.append>|/type/Array#method_append» on it.
 
     my $a;
     say $a.append; # OUTPUT: «[]␤»