vacuumlo: Document data loss risk for unrecognized column types.

Add a caution noting that large object references stored in columns
of types not recognized by vacuumlo (such as text or bigint) will be
treated as orphans and removed, resulting in data loss.  Recommend
the lo extension type to avoid this.

Discussion: https://postgr.es/m/CALsgZNAM=AYK-9ZLR7Z0YLx6Lyx5aSrjjse3T+FmsJ=jTvfhDQ@mail.gmail.com

Author: samimseih

doc/src/sgml/vacuumlo.sgml

@@ -217,6 +217,19 @@
    containing these types. Matching entries are removed from the temporary table.
    The remaining entries in the temporary table identify orphaned LOs.  These are removed.
   </para>
+
+  <caution>
+   <para>
+    Large object references stored in columns of other types, such as
+    <type>text</type> or <type>bigint</type>, are not recognized by
+    <application>vacuumlo</application> and will be treated as orphans,
+    resulting in data loss.  Using the <type>lo</type> type
+    from the <xref linkend="lo"/> extension is recommended for columns that
+    hold large object references, as it allows
+    <application>vacuumlo</application> to find them and the
+    <function>lo_manage</function> trigger to manage them.
+   </para>
+  </caution>
  </refsect1>
 
  <refsect1>