nixos/manual: Document PostgreSQL socket change

This is a backwards-incompatible change and while it won't probably
affect a whole lot of users, it makes sense to give them a heads-up
anyway.

Signed-off-by: aszlig <aszlig@nix.build>

nixos/doc/manual/release-notes/rl-1909.xml

@@ -42,6 +42,27 @@
   </itemizedlist>
  </section>
 
+ <section xmlns="http://docbook.org/ns/docbook"
+         xmlns:xlink="http://www.w3.org/1999/xlink"
+         xmlns:xi="http://www.w3.org/2001/XInclude"
+         version="5.0"
+         xml:id="sec-release-19.09-incompatibilities">
+  <title>Backward Incompatibilities</title>
+
+  <itemizedlist>
+   <listitem>
+    <para>
+     PostgreSQL now uses
+     <filename class="directory">/run/postgresql</filename> as its socket
+     directory instead of <filename class="directory">/tmp</filename>. So
+     if you run an application like eg. Nextcloud, where you need to use
+     the Unix socket path as the database host name, you need to change it
+     accordingly.
+    </para>
+   </listitem>
+  </itemizedlist>
+ </section>
+
  <section xmlns="http://docbook.org/ns/docbook"
          xmlns:xlink="http://www.w3.org/1999/xlink"
          xmlns:xi="http://www.w3.org/2001/XInclude"