pgsql: doc: Clarify pg_get_sequence_data() NULL-return cases

doc: Clarify pg_get_sequence_data() NULL-return cases

The documentation previously said that pg_get_sequence_data() returns
a row of NULL values if the sequence does not exist or if the current
user lacks privileges on it. This was incomplete and could be misleading.
A nonexistent relation name is rejected during regclass input conversion,
while the function returns NULLs for a nonexistent relation OID and
several other cases.

This commit clarifies that the function returns NULLs when the specified
relation OID does not exist, the relation is not a sequence, the current
user lacks SELECT privilege on the sequence, the sequence belongs to
another session's temporary schema, or it is an unlogged sequence on
a standby server.

Author: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAA4eK1JOo0aJRhFHNWpj3hMwaTtNOopY34f1Lh_QD=z=+DrzWQ@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/67490b55f38d98bd08a6fa2764dd44a2a7c5a2db

Modified Files
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doc/src/sgml/func/func-sequence.sgml | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)