Clarifies when allocate was introduced closes #2132

doc/Language/nativecall.pod6

@@ -270,12 +270,22 @@ my $ints = CArray[int32].allocate($number_of_ints); # instantiates an array with
 my $n = get_n_ints($ints, $number_of_ints);
 =end code
 
+I<Note>: C<allocate> was introduced in Rakudo 2018.05. Before that, you had to
+use this mechanism to extend an array to a number of elements:
+
+=begin code :skip-test
+my $ints = CArray[int32].new;
+my $number_of_ints = 10;
+$ints[$number_of_ints - 1] = 0; # extend the array to 10 items
+=end code
+
+
 The memory management of arrays is important to understand. When you create an
 array yourself, then you can add elements to it as you wish and it will be
 expanded for you as required. However, this may result in it being moved in
 memory (assignments to existing elements will never cause this, however). This
-means you'd best know what you're doing if you twiddle with an array after passing
-it to a C library.
+means you'd best know what you're doing if you twiddle with an array after
+passing it to a C library.
 
 By contrast, when a C library returns an array to you, then the memory can not
 be managed by NativeCall, and it doesn't know where the array ends. Presumably,