Link to #minimal-pragma from release notes

(cherry picked from commit 9f607eeb)
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

docs/users_guide/7.8.1-notes.xml

@@ -291,6 +291,7 @@
                 explicitly declare the minimal complete definition of
                 a class. Should an instance not provide the minimal
                 required definitions, a warning will be emitted.
+                See <xref linkend="minimal-pragma"/> for details.
            </para>
        </listitem>