DOCS - Clarify that REFRESH SEQUENCES might be using stale publication data

Today I was experimenting with logical replication of sequences and
encountered some "unexpected" behaviour. It makes sense in hindsight,
but at the time it seemed surprising. It turns out that it is possible
to synchronise sequence values (using "ALTER SUBSCRIPTION... REFRESH
SEQUENCES") even when those sequences are not published.

That's because a subscription only knows which sequences are
subscribed to from the `pg_subscription_rel` table, but that
information can be stale if the publication has since stopped
publishing those sequences. In other words, if a sequence was removed
from the publication but REFRESH PUBLICATION hasn't been run, REFRESH
SEQUENCES would continue to synchronise it since it is still in
`pg_subscription_rel`.

The docs do refer to "currently subscribed sequences" but do not
explicitly warn about this gotcha.

PSA: a patch to add a clarifying sentence.

(I also removed an earlier link to "ALTER SUBSCRIPTION ... REFRESH
PUBLICATION", because it did not seem good to have multiple links to
the same place).

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Kind Regards,
Peter Smith.
Fujitsu Australia