doc: mention psql -l uses the 'postgres' database by default

Reported-by: Mark Wood

Bug: 14912

Discussion: https://postgr.es/m/20171116171735.1474.30450@wrigleys.postgresql.org

Author: David G. Johnston

Backpatch-through: 10

doc/src/sgml/ref/psql-ref.sgml

@@ -283,7 +283,9 @@ EOF
       <listitem>
       <para>
       List all available databases, then exit. Other non-connection
-      options are ignored. This is similar to the meta-command
+      options are ignored. If an explicit database name is not
+      found the <literal>postgres</literal> database, not the user's,
+      will be targeted for connection. This is similar to the meta-command
       <command>\list</command>.
       </para>
       </listitem>