link to Supplier::Preserving, fixes #1610 (#1611)

Author: gfldex

doc/Language/concurrency.pod6

@@ -344,6 +344,9 @@ written as:
         }
     }
 
+A live C<Supply> that keeps values until first tapped can be created with
+L<Supplier::Preserving|/type/Supplier/Preserving>.
+
 An existing supply object can be filtered or transformed, using the methods
 C<grep> and C<map> respectively, to create a new supply in a manner like
 the similarly named list methods: C<grep> returns a supply such that only

doc/Type/Supplier.pod6

@@ -23,7 +23,8 @@ One 42
 Two 42
 
 I<on demand> supplies are created by the factory methods of the L<Supply>
-class or by the C<supply> keyword.
+class or by the C<supply> keyword. A mixture of a live and on-demand C<Supply> can
+be created with a L<Supplier::Preserving|/type/Supplier/Preserving>.
 
 =head1 Methods
 

doc/Type/Supply.pod6

@@ -41,6 +41,9 @@ The L<live method|#method live> returns C<True> on live supplies. Factory
 methods such as L<interval|#method interval>, L<from-list|#method from-list>
 will return I<on demand> supplies.
 
+A live C<Supply> that keeps values until tapped the first time can be created
+with L<Supplier::Preserving|/type/Supplier/Preserving>.
+
 Further examples can be found in the L<concurrency page|/language/concurrency#Supplies>.
 
 =head1 Methods that return Taps