doc: improve tablespace example query and link to helper funcs.

Reported-by: Agustín

Discussion: https://postgr.es/m/172609721070.1128084.6724666076293146476@wrigleys.postgresql.org

Backpatch-through: master

doc/src/sgml/manage-ag.sgml

@@ -530,9 +530,11 @@ CREATE TABLE foo(i int);
    <link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname>
    </link> system catalog, for example
 <synopsis>
-SELECT spcname FROM pg_tablespace;
+SELECT spcname, spcowner::regrole, pg_tablespace_location(oid) FROM pg_tablespace;
 </synopsis>
-   The <xref linkend="app-psql"/> program's <literal>\db</literal> meta-command
+   It is possible to find which databases use which tablespaces;
+   see <xref linkend="functions-info-catalog-table"/>.  The <xref
+   linkend="app-psql"/> program's <literal>\db</literal> meta-command
    is also useful for listing the existing tablespaces.
   </para>
 
@@ -541,10 +543,7 @@ SELECT spcname FROM pg_tablespace;
    point to each of the non-built-in tablespaces defined in the cluster.
    Although not recommended, it is possible to adjust the tablespace
    layout by hand by redefining these links. Under no circumstances perform
-   this operation while the server is running. Note that in PostgreSQL 9.1
-   and earlier you will also need to update the <structname>pg_tablespace</structname>
-   catalog with the new locations. (If you do not, <literal>pg_dump</literal> will
-   continue to output the old tablespace locations.)
+   this operation while the server is running.
   </para>
 
  </sect1>