doc: Clarify what "excluded" represents for INSERT ON CONFLICT

Hi,

Reposting this on its own thread.

https://www.postgresql.org/message-id/flat/CAKFQuwby1aMsJDMeibaBaohgoaZhivAo4WcqHC1%3D9-GDZ3TSng%40mail.gmail.com

As one cannot place excluded in a FROM clause (subquery) in the
ON CONFLICT clause referring to it as a table, with plural rows
nonetheless, leads the reader to infer more about what the
behavior here is than is correct. We already just say use the
table's name for the existing row so just match that pattern
of using the name excluded for the proposed row.

The alias description doesn't have the same issue regarding the
use of the word table and rows, as the use there is more conceptual,
but the wording about "otherwise taken as" is wrong: rather two
labels of excluded end up in scope and you get an ambiguous name error.

The error messages still consider excluded to be a table reference
and this patch does not try to change that. That implementation
detail need not force the user-facing documentation for the feature
to use the term table when it doesn't really apply.

David J.