pgsql: Doc: Improve description around ALTER TABLE ATTACH PARTITION

Doc: Improve description around ALTER TABLE ATTACH PARTITION

This clarifies more how to use and how to take advantage of constraints
when attaching a new partition.

Author: Justin Pryzby
Reviewed-by: Amit Langote, Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/20191028001207.GB23808@telsasoft.com
Backpatch-through: 10

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a7207a24d40e887f08cb8ad11659ecb42e85dead

Modified Files
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doc/src/sgml/ddl.sgml             | 12 ++++++------
doc/src/sgml/ref/alter_table.sgml | 29 +++++++++++++++--------------
2 files changed, 21 insertions(+), 20 deletions(-)