Fix help message and document in pg_receivexlog.

Add SLOTNAME placeholder to --slot option in help message and
document.

doc/src/sgml/ref/pg_receivexlog.sgml

@@ -227,7 +227,7 @@ PostgreSQL documentation
      </varlistentry>
 
      <varlistentry>
-      <term><option>--slot</option></term>
+      <term><option>--slot=<replaceable class="parameter">slotname</replaceable></option></term>
       <listitem>
         <para>
          Require <application>pg_receivexlog</application> to use an existing

src/bin/pg_basebackup/pg_receivexlog.c

@@ -74,7 +74,7 @@ usage(void)
 	printf(_("  -U, --username=NAME    connect as specified database user\n"));
 	printf(_("  -w, --no-password      never prompt for password\n"));
 	printf(_("  -W, --password         force password prompt (should happen automatically)\n"));
-	printf(_("      --slot             replication slot to use\n"));
+	printf(_("      --slot=SLOTNAME    replication slot to use\n"));
 	printf(_("\nReport bugs to <pgsql-bugs@postgresql.org>.\n"));
 }