NEWS, man: move description of SR-IOV-R net naming to v255

https://github.com/systemd/systemd/pull/29582 adds the "v254" name. This also
changes what the default is and what "latest" refers to. Without the name, the
code could be enabled via runtime configuration. Nevertheless, it could be
enabled at compilation time. In other words:
  meson setup build -Ddefault-net-naming-scheme=v254
would work, but
  net.naming-scheme=v254
would fail.

It is possible that people were using the compile-time override, so I think
we should allow "v254" scheme to stay and clearly document that it wasn't the
default.

Unfortunately, unless people manually introduced the compile-time override, we
were never actually testing the new code too. So all the pull request testing
was not useful.

man/systemd.net-naming-scheme.xml

@@ -505,13 +505,23 @@
         <varlistentry>
           <term><constant>v254</constant></term>
 
-          <listitem><para>Naming was changed for SR-IOV virtual device representors.</para>
+          <listitem><para>Naming was changed for SR-IOV virtual device representors, optionally settable at
+          compilation time. The <literal>r<replaceable>slot</replaceable></literal> suffix was added to
+          differentiate SR-IOV virtual device representors attached to a single physical device interface.
+          Because of a mistake, this scheme was <emphasis>not the the default scheme for systemd version
+          254</emphasis>.</para>
 
-          <para>The <literal>r<replaceable>slot</replaceable></literal> suffix was added to differentiate SR-IOV
-          virtual device representors attached to a single physical device interface.
-          </para>
+          <xi:include href="version-info.xml" xpointer="v255"/>
+          </listitem>
+        </varlistentry>
 
-          <xi:include href="version-info.xml" xpointer="v254"/>
+        <varlistentry>
+          <term><constant>v255</constant></term>
+
+          <listitem><para>Naming was changed for SR-IOV virtual device representors to enable the
+          change introduced in <constant>v254</constant> by default.</para>
+
+          <xi:include href="version-info.xml" xpointer="v255"/>
           </listitem>
         </varlistentry>