man: add a shared "experimental" blurb for features

(cherry picked from commit a6eb2b4d2e7f747bbfb0f5f3ebb1421855113d36)

man/experimental.xml

@@ -17,6 +17,10 @@
   They will be stabilized and given a backward-compatibility guarantee in a future release,
   but may also possibly be removed.</para>
 
+  <para id="feature">This feature is experimental and subject to change without advance notice.
+  It will be stabilized and given a backward-compatibility guarantee in a future release,
+  but may also possibly be removed.</para>
+
   <!-- for a whole manpage -->
   <refsect1 id="component"><title>Experimental</title>
     <para>This component is experimental and subject to change without advance notice.

man/loader.conf.xml

@@ -435,8 +435,7 @@ sbvarsign --attr "${attr}" --key KEK.key --cert KEK.pem --output db.auth db db.e
       <varlistentry>
         <term>reboot-for-bitlocker</term>
 
-        <listitem><para>Caveat: This feature is experimental, and is likely to be changed (or removed in its
-        current form) in a future version of systemd.</para>
+        <listitem><xi:include href="experimental.xml" xpointer="feature" />
 
         <para>Work around BitLocker requiring a recovery key when the boot loader was updated (disabled by
         default).</para>

man/systemd-vmspawn.xml

@@ -355,8 +355,7 @@
         <varlistentry>
           <term><option>--coco=</option></term>
 
-          <listitem><para>Caveat: This feature is experimental, and is likely to be changed (or removed in
-          its current form) in a future version of systemd.</para>
+          <listitem><xi:include href="experimental.xml" xpointer="feature" />
 
           <para>Configures whether to run the guest as a confidential VM. Takes one of
           <literal>no</literal>, <literal>sev-snp</literal> or <literal>tdx</literal>. Defaults to