Docs: Always use true|false instead of sometimes on|off for the subscription options

Hi hackers,

There are lots of subscription options listed on the CREATE
SUBSCRIPTION page [1].

Although these boolean options are capable of accepting different
values like "1|0", "on|off", "true|false", here they are all described
only using values "true|false".

~

IMO this consistent way of describing the boolean option values ought
to be followed also on the ALTER SUBSCRIPTION page [2]. Specifically,
the ALTER SUBSCRIPTION page has one mention of "SET (failover =
on|off)" which I think should be changed to say "SET (failover =
true|false)"

Now this little change hardly seems important, but actually, it is
motivated by another thread [3] under development where this ALTER
SUBSCRIPTION "(failover = on|off)" was copied again, thereby making
the consistency between the CREATE SUBSCRIPTION and ALTER SUBSCRIPTION
pages grow further apart, so I think it is best to nip that in the bud
and simply use "true|false" values everywhere.

PSA a patch to make this proposed change.

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[1] https://www.postgresql.org/docs/devel/sql-createsubscription.html
[2] https://www.postgresql.org/docs/devel/sql-altersubscription.html
[3] https://www.postgresql.org/message-id/flat/8fab8-65d74c80-1-2f28e880%4039088166

Kind Regards,
Peter Smith.
Fujitsu Australia