tree-wide: reference UAPI.20 spec that has been published for Varlink now

This updates a couple of links talking about Varlink:

1. anything that refers to specific Varlink concepts now point to the
   UAPI.20 spec

2. More general references to Varlink as a whole point to both the
   varlink.org domain and the spec.

man/sd-varlink.xml

@@ -36,8 +36,10 @@
 
     <para><filename>sd-varlink.h</filename> is part of
     <citerefentry><refentrytitle>libsystemd</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
-    provides APIs for implementing Varlink IPC clients and services. See <ulink url="https://varlink.org/"/>
-    for more information about Varlink IPC.</para>
+    provides APIs for implementing Varlink IPC clients and services. See <ulink
+    url="https://varlink.org/">Varlink IPC</ulink> for more information about Varlink IPC, as well as the <ulink
+    url="https://uapi-group.org/specifications/specs/varlink/">UAPI.20 Varlink IPC</ulink>
+    specification.</para>
 
     <para>Varlink IPC uses <ulink url="https://json.org/">JSON</ulink> as marshalling format. The sd-varlink
     API relies on the

man/varlinkctl.xml

@@ -105,7 +105,9 @@
     <title>Description</title>
 
     <para><command>varlinkctl</command> may be used to introspect and invoke <ulink
-    url="https://varlink.org/">Varlink</ulink> services.</para>
+    url="https://varlink.org/">Varlink IPC</ulink> services, following the <ulink
+    url="https://uapi-group.org/specifications/specs/varlink/">UAPI.20 Varlink IPC</ulink>
+    specification.</para>
 
     <para>Services are referenced by one of the following:</para>