PGDOCS - add more links in the pub/sub reference pages

The "Description" and "Notes" parts of the following logical
replication PUB/SUB reference pages (almost always) link to each other
whenever a sibling command gets mentioned.

CREATE PUBLICATION
ALTER PUBLICATION
DROP PUBLICATION

CREATE SUBSCRIPTION
ALTER SUBSCRIPTION
DROP SUBSCRIPTION

~

AFAICT the only omissions are:

ALTER PUBLICATION page -- mentions ALTER SUBSCRIPTION but there is no link
DROP SUBSCRIPTION page -- mentions ALTER SUBSCRIPTION but there is no link

~

Here is a patch to add the 2 missing references:

ref/alter_subscription.sgml ==> added more ids
ref/alter_publication.sgml ==> added link to
"sql-altersubscription-refresh-publication"
ref/drop_subscription.sgml ==> added link to "sql-altersubscription"

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Kind Regards,
Peter Smith.
Fujitsu Australia