Improve documentation for START_REPLICATION ... LOGICAL.

The starting point may not be exactly what the client requested; it
may be at the slot's confirmed_flush_lsn.

Also, upgrade the message from DEBUG1 to LOG when this happens.

Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/c5c861d576f2511732f8002c76245da587110b1c.camel%40j-davis.com

doc/src/sgml/protocol.sgml

@@ -2410,10 +2410,22 @@ The commands accepted in replication mode are:
     <term><literal>START_REPLICATION</literal> <literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable> <literal>LOGICAL</literal> <replaceable class="parameter">XXX/XXX</replaceable> [ ( <replaceable>option_name</replaceable> [ <replaceable>option_value</replaceable> ] [, ...] ) ]</term>
     <listitem>
      <para>
-      Instructs server to start streaming WAL for logical replication, starting
-      at WAL location <replaceable class="parameter">XXX/XXX</replaceable>. The server can
-      reply with an error, for example if the requested section of WAL has already
-      been recycled. On success, server responds with a CopyBothResponse
+      Instructs server to start streaming WAL for logical replication,
+      starting at either WAL location <replaceable
+      class="parameter">XXX/XXX</replaceable> or the slot's
+      <literal>confirmed_flush_lsn</literal> (see <xref
+      linkend="view-pg-replication-slots"/>), whichever is greater. This
+      behavior makes it easier for clients to avoid updating their local LSN
+      status when there is no data to process. However, starting at a
+      different LSN than requested might not catch certain kinds of client
+      errors; so the client may wish to check that
+      <literal>confirmed_flush_lsn</literal> matches its expectations before
+      issuing <literal>START_REPLICATION</literal>.
+     </para>
+
+     <para>
+      The server can reply with an error, for example if the
+      slot does not exist. On success, server responds with a CopyBothResponse
       message, and then starts to stream WAL to the frontend.
      </para>
 

src/backend/replication/logical/logical.c

@@ -520,8 +520,12 @@ CreateDecodingContext(XLogRecPtr start_lsn,
 		 * xlog records didn't result in anything relevant for logical
 		 * decoding. Clients have to be able to do that to support synchronous
 		 * replication.
+		 *
+		 * Starting at a different LSN than requested might not catch certain
+		 * kinds of client errors; so the client may wish to check that
+		 * confirmed_flush_lsn matches its expectations.
 		 */
-		elog(DEBUG1, "cannot stream from %X/%X, minimum is %X/%X, forwarding",
+		elog(LOG, "%X/%X has been already streamed, forwarding to %X/%X",
 			 LSN_FORMAT_ARGS(start_lsn),
 			 LSN_FORMAT_ARGS(slot->data.confirmed_flush));