Expain that using a PIM will fail until Tails 4.0

wiki/src/doc/encryption_and_privacy/veracrypt.mdwn

@@ -78,6 +78,13 @@ volume was created:
   [See the <span class="application">VeraCrypt</span> documentation on
   PIM.](https://www.veracrypt.fr/en/Personal%20Iterations%20Multiplier%20\(PIM\).html)
 
+<div class="bug">
+
+<p>Due to current limitations in Debian, using a PIM fails in Tails. It
+will become possible in Tails 4.0 (late 2019).</p>
+
+</div>
+
 - **Hidden volume**: if you want to unlock the hidden volume inside the
   <span class="application">VeraCrypt</span> volume.