Expands the "fixed size array" section

Indexing it also as shaped arrays, expanding a bit the examples, and
adding the changes in 2018.09. Closes #2326

doc/Language/list.pod6

@@ -591,33 +591,48 @@ an undefined C<Any>:
     @n.default.perl.say;            # OUTPUT: «Real␤»
     @n[0].say;                      # OUTPUT: «(Real)␤»
 
+X<|Shaped arrays>
 =head2 Fixed size arrays
 
-To limit the dimensions of an C<Array> provide the dimensions separated by C<,>
-or C<;> in brackets after the name of the array container. The values of such
-an C<Arrays> will default to C<Any>. The shape can be accessed at runtime via
-the C<shape> method.
+To limit the dimensions of an C<Array>, provide the dimensions separated by C<,>
+or C<;> in brackets after the name of the array container in case there is more
+than one dimension; these are called I<shaped> arrays too. The values of such a
+kind of C<Array> will default to C<Any>. The shape can be accessed at runtime
+via the C<shape> method.
 
     my @a[2,2];
     say @a.perl;
     # OUTPUT: «Array.new(:shape(2, 2), [Any, Any], [Any, Any])␤»
-    say @a.shape;
-    # OUTPUT: «(2 2)␤»
+    say @a.shape;         # OUTPUT: «(2 2)␤»
+    my @just-three[3] = <alpha betta kappa>;
+    say @just-three.perl;
+    # OUTPUT: «Array.new(:shape(3,), ["alpha", "betta", "kappa"])␤»
 
-Assignment to a fixed size Array will promote a List of Lists to an Array of
-Arrays.
+Shape will control the amount of elements that can be assigned by dimension:
+
+=begin code :skip-test<Illustrates exception>
+my @just-two[2] = <alpha betta kappa>;
+# Will throw exception: «Index 2 for dimension 1 out of range (must be 0..1)»
+=end code
+
+Assignment to a fixed size C<Array> will promote a List of Lists to an Array of
+Arrays (making then mutable in the process).
 
     my @a[2;2] = (1,2; 3,4);
+    say @a.Array; # OUTPUT: «[1 2 3 4]␤»
     @a[1;1] = 42;
     say @a.perl;
     # OUTPUT: «Array.new(:shape(2, 2), [1, 2], [3, 42])␤»
 
+As the third statement shows, you can assign directly to an element in a shaped
+array too. B<Note>: the second statement works only from version 2018.09.
+
 =head2 Itemization
 
-For most uses, Arrays consist of a number of slots each containing a C<Scalar>
-of the correct type.  Each such C<Scalar>, in turn, contains a value of that
-type. Perl 6 will automatically type-check values and create Scalars to contain
-them when Arrays are initialized, assigned to, or constructed.
+For most uses, C<Array>s consist of a number of slots each containing a
+C<Scalar> of the correct type.  Every such C<Scalar>, in turn, contains a value
+of that type. Perl 6 will automatically type-check values and create Scalars to
+contain them when Arrays are initialized, assigned to, or constructed.
 
 This is actually one of the trickiest parts of Perl 6 list handling to get a
 firm understanding of.