pgsql: docs: Consistently use to indicate optional parameter

docs: Consistently use <optional> to indicate optional parameters

Some functions were using square brackets instead, replace them all
with <optional>.

Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: jian he <jian.universality@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CACJufxFfUbSph5UUSsZbL4SitbuPuW%3DEccpKgEaZrjtrPPuadQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0d829703363ba70844ff545ccc73befb2a7fed2e

Modified Files
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doc/src/sgml/func.sgml | 48 ++++++++++++++++++++++++------------------------
1 file changed, 24 insertions(+), 24 deletions(-)