Document that tar archives are now properly terminated.

Commit 5a1007a508 changed the server
behavior, but I didn't notice that the existing behavior was
documented, and therefore did not update the documentation.
This commit does that.

I chose to mention that the behavior has changed rather than just
removing the reference to a deviation from a standard. It seemed
like that might be helpful to tool authors.

Discussion: http://postgr.es/m/CA+TgmoaYZbz0=Yk797aOJwkGJC-LK3iXn+wzzMx7KdwNpZhS5g@mail.gmail.com

doc/src/sgml/protocol.sgml

@@ -2809,8 +2809,10 @@ The commands accepted in replication mode are:
       than <literal>pg_default</literal> and <literal>pg_global</literal>. The data in
       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.
+      standard) dump of the tablespace contents. Prior to
+      <literal>PostgreSQL</literal> 15, the server omitted the two trailing
+      blocks of zeroes specified in the standard, but this is no longer the
+      case.
       After the tar data is complete, and if a backup manifest was requested,
       another CopyOutResponse result is sent, containing the manifest data for the
       current base backup. In any case, a final ordinary result set will be