Add an admonition against uncontainerized binding to array elements

lib/Language/containers.pod

@@ -183,6 +183,20 @@ The answer is that binding to array elements is recognized at the syntax
 level and instead of emitting code for a normal binding operation, a special
 method on the array is called that knows how to do the binding itself.
 
+Note that, while supported, one should generally avoid directly binding
+uncontainerized things into array elements.  Doing so may produce
+counter-intuitive results when the array is used later.
+
+    my @a = (1,2,3);
+    @a[0] := 42;         # This is not recommended, use assignment instead.
+    my $b := 42;
+    @a[1] := $b;         # Nor is this.
+    @a[2] = $b;          # ...but this is fine.
+    @a[1,2] := 1,2;      # This is not allowed and will fail.
+
+Operations that mix Lists and Arrays generally protect against such
+a thing happening accidentally.
+
 =head2 Flattening, items and containers
 
 The C<%> and C<@> sigils in Perl 6 generally indicate flattening. Hence