Documenation fixups for replication protocol.

There is no CopyResponse message; it should be CopyOutResponse.

Also, if there is no WAL to stream, the server does not immediately
send a CommandComplete; it's a historical timeline, so it will send a
response tuple first.

Discussion: https://postgr.es/m/0a2c985ebcaa1acd385350aeba561b6509187394.camel@j-davis.com

doc/src/sgml/protocol.sgml

@@ -2065,8 +2065,8 @@ The commands accepted in replication mode are:
       the history of the server, the server will stream all the WAL on that
       timeline starting from the requested start point up to the point where
       the server switched to another timeline. If the client requests
-      streaming at exactly the end of an old timeline, the server responds
+      streaming at exactly the end of an old timeline, the server skips COPY
-      immediately with CommandComplete without entering COPY mode.
+      mode entirely.
      </para>
 
      <para>
@@ -2638,7 +2638,7 @@ The commands accepted in replication mode are:
      </para>
      <para>
       When the backup is started, the server will first send two
-      ordinary result sets, followed by one or more CopyResponse
+      ordinary result sets, followed by one or more CopyOutResponse
       results.
      </para>
      <para>
@@ -2681,15 +2681,15 @@ The commands accepted in replication mode are:
       </variablelist>
      </para>
      <para>
-      After the second regular result set, one or more CopyResponse results
+      After the second regular result set, one or more CopyOutResponse results
       will be sent, one for the main data directory and one for each additional tablespace other
       than <literal>pg_default</literal> and <literal>pg_global</literal>. The data in
-      the CopyResponse results will be a tar format (following the
+      the CopyOutResponse results will be a tar format (following the
       <quote>ustar interchange format</quote> specified in the POSIX 1003.1-2008
       standard) dump of the tablespace contents, except that the two trailing
       blocks of zeroes specified in the standard are omitted.
       After the tar data is complete, and if a backup manifest was requested,
-      another CopyResponse result is sent, containing the manifest data for the
+      another CopyOutResponse result is sent, containing the manifest data for the
       current base backup. In any case, a final ordinary result set will be
       sent, containing the WAL end position of the backup, in the same format as
       the start position.