Doc: you must own the target object to use SECURITY LABEL.

For some reason this wasn't mentioned before.

Author: Patrick Stählin <me@packi.ch>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/931e012a-57ba-41ba-9b88-24323a46dec5@packi.ch
Backpatch-through: 13

doc/src/sgml/ref/security_label.sgml

@@ -84,6 +84,10 @@ SECURITY LABEL [ FOR <replaceable class="parameter">provider</replaceable> ] ON
    based on object labels, rather than traditional discretionary access control
    (DAC) concepts such as users and groups.
   </para>
+
+  <para>
+   You must own the database object to use <command>SECURITY LABEL</command>.
+  </para>
  </refsect1>
 
  <refsect1>