pgsql: Document partitiong tables ancillary object handling some more

Document partitiong tables ancillary object handling some more

Add a couple of lines to make it explicit that indexes, constraints,
triggers are added, removed, or left alone.

Backpatch to pg11.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20200421162038.GA18628@alvherre.pgsql

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cda02408f20dda90dfe4eb8b6a1adf053afe7446

Modified Files
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doc/src/sgml/ref/alter_table.sgml  | 9 ++++++---
doc/src/sgml/ref/create_table.sgml | 4 +++-
2 files changed, 9 insertions(+), 4 deletions(-)