Improve documentation about binary/textual output mode for output plugins.

Also improve related error message as it contributed to the confusion.

Discussion: CAB7nPqQrqFzjqCjxu4GZzTrD9kpj6HMn9G5aOOMwt1WZ8NfqeA@mail.gmail.com,
    CAB7nPqQXc_+g95zWnqaa=mVQ4d3BVRs6T41frcEYi2ocUrR3+A@mail.gmail.com

Per discussion between Michael Paquier, Robert Haas and Andres Freund

Backpatch to 9.4 where logical decoding was introduced.

contrib/test_decoding/expected/binary.out

@@ -14,7 +14,7 @@ SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'for
 
 -- fails, binary plugin, textual consumer
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'force-binary', '1');
-ERROR:  output plugin cannot produce binary output
+ERROR:  logical decoding output plugin "test_decoding" produces binary output, but "pg_logical_slot_get_changes(name,pg_lsn,integer,text[])" expects textual data
 -- succeeds, textual plugin, binary consumer
 SELECT data FROM pg_logical_slot_get_binary_changes('regression_slot', NULL, NULL, 'force-binary', '0');
  data 

doc/src/sgml/logicaldecoding.sgml

@@ -355,6 +355,24 @@ CREATE TABLE another_catalog_table(data text) WITH (user_catalog_table = true);
     </para>
    </sect2>
 
+   <sect2 id="logicaldecoding-output-mode">
+    <title>Output Modes</title>
+    <para>
+     Output plugin callbacks can pass data to the consumer in nearly arbitrary
+     formats. For some use cases, like viewing the changes via SQL, returning
+     data in a datatype that can contain arbitrary data (i.e. bytea) is
+     cumbersome. If the output plugin only outputs textual data in the
+     server's encoding it can declare that by
+     setting <literal>OutputPluginOptions.output_mode</>
+     to <literal>OUTPUT_PLUGIN_TEXTUAL_OUTPUT</> instead
+     of <literal>OUTPUT_PLUGIN_BINARY_OUTPUT</> in
+     the <link linkend="logicaldecoding-output-plugin-startup">startup
+     callback</>. In that case all the data has to be in the server's encoding
+     so a <type>text</> can contain it. This is checked in assertion enabled
+     builds.
+    </para>
+   </sect2>
+
    <sect2 id="logicaldecoding-output-plugin-callbacks">
     <title>Output Plugin Callbacks</title>
     <para>
@@ -405,7 +423,8 @@ typedef struct OutputPluginOptions
 </programlisting>
       <literal>output_type</literal> has to either be set to
       <literal>OUTPUT_PLUGIN_TEXTUAL_OUTPUT</literal>
-      or <literal>OUTPUT_PLUGIN_BINARY_OUTPUT</literal>.
+      or <literal>OUTPUT_PLUGIN_BINARY_OUTPUT</literal>. See also
+      <xref linkend="logicaldecoding-output-mode"/>.
      </para>
      <para>
       The startup callback should validate the options present in

src/backend/replication/logical/logicalfuncs.c

@@ -401,7 +401,9 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 			ctx->options.output_type != OUTPUT_PLUGIN_TEXTUAL_OUTPUT)
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("output plugin cannot produce binary output")));
+					 errmsg("logical decoding output plugin \"%s\" produces binary output, but \"%s\" expects textual data",
+							NameStr(MyReplicationSlot->data.plugin),
+							format_procedure(fcinfo->flinfo->fn_oid))));
 
 		ctx->output_writer_private = p;