doc: clarify the behavior of unopenable listen_addresses

Reported-by: Gurjeet Singh

Discussion: https://postgr.es/m/CABwTF4WYPD9ov-kcSq1+J+ZJ5wYDQLXquY6Lu2cvb-Y7pTpSGA@mail.gmail.com

Backpatch-through: 11

doc/src/sgml/config.sgml

@@ -656,10 +656,15 @@ include_dir 'conf.d'
          <literal>::</literal> allows listening for all IPv6 addresses.
          If the list is empty, the server does not listen on any IP interface
          at all, in which case only Unix-domain sockets can be used to connect
-         to it.
+         to it.  If the list is not empty, the server will start if it
+         can listen on at least one TCP/IP address.  A warning will be
+         emitted for any TCP/IP address which cannot be opened.
          The default value is <systemitem class="systemname">localhost</systemitem>,
          which allows only local TCP/IP <quote>loopback</quote> connections to be
-         made.  While client authentication (<xref
+         made.
+       </para>
+       <para>
+         While client authentication (<xref
          linkend="client-authentication"/>) allows fine-grained control
          over who can access the server, <varname>listen_addresses</varname>
          controls which interfaces accept connection attempts, which