doc: Document default values for some pg_recvlogical options.

The documentation did not previously mention the default values for
the --fsync-interval and --plugin options, even though pg_recvlogical --help
shows them. This omission made it harder for users to understand
the tool's behavior from the documentation alone.

This commit adds the missing default value descriptions for both options
to the pg_recvlogical documentation.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://postgr.es/m/CAHGQGwFqssPBjkWMFofGq32e_tANOeWN-cM=6biAP3nnFUXMRw@mail.gmail.com

Author: MasaoFujii

doc/src/sgml/ref/pg_recvlogical.sgml

@@ -206,7 +206,7 @@ PostgreSQL documentation
        <para>
         Specifies how often <application>pg_recvlogical</application> should
         issue <function>fsync()</function> calls to ensure the output file is
-        safely flushed to disk.
+        safely flushed to disk. The default value is 10 seconds.
        </para>
 
        <para>
@@ -275,8 +275,10 @@ PostgreSQL documentation
       <listitem>
        <para>
         When creating a slot, use the specified logical decoding output
-        plugin. See <xref linkend="logicaldecoding"/>. This option has no
-        effect if the slot already exists.
+        plugin. See <xref linkend="logicaldecoding-output-plugin"/> for
+        information about the plugins <productname>PostgreSQL</productname>
+        provides. The default is <xref linkend="test-decoding"/>.
+        This option has no effect if the slot already exists.
        </para>
       </listitem>
      </varlistentry>