Add note about hidepid mount options

Closes: #925552

en/issues.dbk

@@ -39,6 +39,22 @@ information mentioned in <xref linkend="morereading"/>.
     </para>
   </section>
 
+  <section id="hidepid-unsupported">
+    <!-- stretch to buster-->
+    <title>Hidepid mount options for procfs unsupported</title>
+    <para>
+      The hidepid mount options to <filename>/proc</filename> are known to cause
+      problems with current versions of systemd, and are considered by systemd
+      upstream to be an unsupported configuration. Users who have modified
+      <filename>/etc/fstab</filename> to enable these options are advised to
+      disable them before the upgrade, to ensure login sessions work on
+      &releasename;. (A possible route to re-enabling them is outlined on the
+      wiki's <ulink
+      url="https://wiki.debian.org/Hardening#Mounting_.2Fproc_with_hidepid">Hardening</ulink>
+      page.)
+    </para>
+  </section>
+
   <section id="noteworthy-obsolete-packages" condition="fixme">
     <title>Noteworthy obsolete packages</title>
     <para>