Enhance create subscription reference manual

Currently the manual explains create subscription's "failover"
parameter as follows:

<para>
Since no connection is made when this option is
<literal>false</literal>, no tables are subscribed. To initiate
replication, you must manually create the replication slot, enable
the failover if required, enable the subscription, and refresh the
subscription. See
<xref linkend="logical-replication-subscription-examples-deferred-slot"/>
for examples.
</para>

While translating it into Japanese, we were little confused what "the
failover" actually means because we thought it might refer to the
failover operation or the failover parameter in the command. After a
discussion in the translation team, we concluded that it must refer to
the failover parameter. If we were correct, it would be nice to add
<literal> tag to "failover" to make it clear that it refers to a
failover parameter. Attached is the patch to do that.

Best reagards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp