DOCS: Missing tags for some SEQUENCE fields

Hi,

While reviewing the recent patches for SEQUENCE documentation I found
[1] a few more instances where the <structfield> tag should have been
used for some of the sequence fields (per the recent push [2]).

PSA patch v1 to address these.

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[1] Searched all docs for 'is_called' and for 'last_value'
[2] https://github.com/postgres/postgres/commit/980a855c5c2e21e964a739694e24004f72e03fdf

Kind Regards,
Peter Smith.
Fujitsu Australia