doc: Document reopen of output file via SIGHUP in pg_recvlogical.

When pg_recvlogical receives a SIGHUP signal, it closes the current
output file and reopens a new one. This is useful since it allows us to
rotate the output file by renaming the current file and sending a SIGHUP.

This behavior was previously undocumented. This commit adds
the missing documentation.

Back-patch to all supported versions.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Shinya Kato <shinya11.kato@gmail.com>
Discussion: https://postgr.es/m/0977fc4f-1523-4ecd-8a0e-391af4976367@oss.nttdata.com
Backpatch-through: 13

doc/src/sgml/ref/pg_recvlogical.sgml

@@ -46,6 +46,16 @@ PostgreSQL documentation
     a slot without consuming it, use
    <link linkend="functions-replication"><function>pg_logical_slot_peek_changes</function></link>.
   </para>
+
+  <para>
+   When <application>pg_recvlogical</application> receives
+   a <systemitem>SIGHUP</systemitem> signal, it closes the current output file
+   and opens a new one using the filename specified by
+   the <option>--file</option> option.  This allows us to rotate
+   the output file by first renaming the current file and then sending
+   a <systemitem>SIGHUP</systemitem> signal to
+   <application>pg_recvlogical</application>.
+  </para>
  </refsect1>
 
  <refsect1>