pgsql: Doc: clarify behavior of boolean options in replication commands

Doc: clarify behavior of boolean options in replication commands.

defGetBoolean() allows the "value" part of "option = value"
syntax to be omitted, in which case it's taken as "true".
This is acknowledged in our syntax summaries for relevant commands,
but we don't seem to have documented the actual behavior anywhere.
Do so for CREATE/ALTER PUBLICATION/SUBSCRIPTION. Use generic
boilerplate text for this, with the idea that we can copy-and-paste
it into other relevant reference pages, whenever someone gets
around to that.

Peter Smith, edited a bit by me

Discussion: https://postgr.es/m/CAHut+PvwjZfdGt2R8HTXgSZft=jZKymrS8KUg31pS7zqaaWKKw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ec7e053a98f39a9e3c7e6d35f0d2e83933882399

Modified Files
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doc/src/sgml/ref/alter_subscription.sgml | 7 +++++++
doc/src/sgml/ref/create_publication.sgml | 7 +++++++
doc/src/sgml/ref/create_subscription.sgml | 7 +++++++
3 files changed, 21 insertions(+)