Bring prose about Array constructor up to date

Fixes #1229

doc/Language/syntax.pod6

@@ -442,15 +442,20 @@ inside:
     say ['a', 'b', 42].join(' ');   # OUTPUT: «a b 42␤»
     #   ^^^^^^^^^^^^^^ Array constructor
 
-The array constructor flattens non-itemized arrays and lists, but not itemized
-arrays themselves:
+If the constructor is given a single L<Iterable>, it'll clone and flatten it.
+If you want an C<Array> with just 1 element that is that C<Iterable>, ensure
+to use a comma after it:
 
     my @a = 1, 2;
-    # flattens:
-    say [@a, 3, 4].elems;       # OUTPUT: «4␤»
+    say [@a].perl;  # OUTPUT: «[1, 2]␤»
+    say [@a,].perl; # OUTPUT: «[[1, 2],]␤»
 
-    # does not flatten:
-    say [[@a], [3, 4]].elems;   # OUTPUT: «2␤»
+The C<Array> constructor does not flatten other types of contents.
+Use the L<Slip> prefix operator (C<|>) to flatten the needed items:
+
+    my @a = 1, 2;
+    say [@a, 3, 4].perl;  # OUTPUT: «[[1, 2], 3, 4]␤»
+    say [|@a, 3, 4].perl; # OUTPUT: «[1, 2, 3, 4]␤»
 
 =head3 Hash literals