pgsql: Doc: back-patch documentation about limitations of CHECK constra

Doc: back-patch documentation about limitations of CHECK constraints.

Back-patch commits 36d442a25 and 1f66c657f into all supported
branches. I'd considered doing this when putting in the latter
commit, but failed to pull the trigger. Now that we've had an
actual field complaint about the lack of such docs, let's do it.

Per bug #16158 from Piotr Jander. Original patches by Lætitia Avrot,
Patrick Francelle, and me.

Discussion: https://postgr.es/m/[email protected]

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7ef59c9b183955e841c39c227e9ebc5c49eb9f18

Modified Files
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doc/src/sgml/ddl.sgml | 53 +++++++++++++++++++++++++++
doc/src/sgml/ref/alter_domain.sgml | 21 +++++++++--
doc/src/sgml/ref/alter_table.sgml | 71 ++++++++++++++++++++++++-------------
doc/src/sgml/ref/create_domain.sgml | 24 +++++++++++++
doc/src/sgml/ref/create_table.sgml | 3 +-
5 files changed, 144 insertions(+), 28 deletions(-)