pgsql: doc: Mention pg_get_partition_constraintdef()

doc: Mention pg_get_partition_constraintdef()

All the other SQL functions reconstructing definitions or commands are
listed in the documentation, except this one.

Oversight in 1848b73d4576.

Author: Todd Liebenschutz-Jones <todd.liebenschutz-jones@starlingbank.com>
Discussion: https://postgr.es/m/CAGTRfaD6uRQ9iutASDzc_iDoS25sQTLWgXTtR3ta63uwTxq6bA@mail.gmail.com
Backpatch-through: 14

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/83172d94164a6c327d7306dd9e0906293fd58ca2

Modified Files
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doc/src/sgml/func.sgml | 15 +++++++++++++++
1 file changed, 15 insertions(+)