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