Add glossary entry, and link references, closes Raku#4018

doc/Language/glossary.pod6

@@ -330,6 +330,26 @@ A document originally written by L<#TheDamian>, in which he tried to explain
 the L<Apocalypses|#Apocalypse> to the common (wo)man. Now only kept as an
 historical document for reference. See also L<#Synopsis>.
 
+X<|forward declaration>
+=head1 Forward declarations
+
+Declare the scope and/or type of a functional entity (class or routine)
+without actually declaring its code by using
+the L<"stub" operator|/language/operators#index-entry-..._operators>; code
+will be declared later on in the same file.
+
+=for code
+grammar Quux {
+    my regex future {...};
+
+    say "foobarbaz" ~~ /<future>/;
+
+    regex future { f \w** 2 b \w ** 2 }
+}
+
+In this case, the regex acts as a method; please note that the scope is only
+declared once.
+
 X<|fiddly>
 =head1 fiddly
 

doc/Language/typesystem.pod6

@@ -128,8 +128,8 @@ automatically if no such scope exists already.
     put Foo::Bar::.keys;
     # OUTPUT: «C␤»
 
-X«|... (forward declaration)»
-X«Forward declarations» can be provided with a block containing only C<...>. The
+X«Forward type declarations» can be provided with a block containing only C<...>,
+the L<"stub" operator|/language/operators#index-entry-..._operators>. The
 compiler will check at the end of the current scope if the type is defined.
 
     class C {...}