Adds constant prefix to the Variables page

This closes #1566 although there was a pretty good section on the
Terms page, for some reason. I have added just a simple example and
linked the previous page, there's no need for duplicates.

doc/Language/5to6-nutshell.pod6

@@ -1213,8 +1213,8 @@ constant DEBUG = 0;      # Perl 6
 =for code :lang<perl5>
 use constant pi => 4 * atan2(1, 1); # Perl 5
 =for code
- tau, pi, e, i; # built-in constants in Perl 6
- τ, π, 𝑒        # and their unicode equivalents
+tau, pi, e, i; # built-in constants in Perl 6
+τ, π, 𝑒        # and their unicode equivalents
 
 =head3 C<encoding>
 

doc/Language/variables.pod6

@@ -864,6 +864,19 @@ stay as 84 because the block returns a defined value (C<say> returns
 true). Otherwise the C<die> statement will cause the block to exit
 unsuccessfully, resetting the answer to 42.
 
+X<|constant (Prefix)>
+=head2 The C<constant> prefix
+
+The C<constant> prefix declares that a container value is not going to change
+during its lifetime.
+
+=begin code :skip-test<Illustrating error>
+constant $pi2 = pi * 2;
+$pi2 = 6; # OUTPUT: «(exit code 1) Cannot assign to an immutable value␤
+=end code
+
+The value is assigned at compile time. Please check L<the section on constants in the Terms page|/language/terms#Constants> for additional information.
+
 =head1 Type constraints and initialization
 
 Variables have a type constraint via the L<container|/language/containers> they