pgsql: doc: Document more that relations share a namespace

doc: Document more that relations share a namespace

This was already documented in the CREATE INDEX reference, but not in
the introductory "Data Definition" chapter. Also, document that the
index that implements a constraint has the same name as the
constraint.

Author: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
Reviewed-by: jian he <jian(dot)universality(at)gmail(dot)com>
Reviewed-by: David G. Johnston <david(dot)g(dot)johnston(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/CACJufxFG682tYcP9aH_F-jrqq5End8MHZR77zcp1%3DDUrEsSu1Q%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d0283387d358e5e0a64a439b7fe906399b59fdd9

Modified Files
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doc/src/sgml/ddl.sgml | 6 +++++-
doc/src/sgml/ref/create_table.sgml | 8 +++++---
2 files changed, 10 insertions(+), 4 deletions(-)