pgsql: doc: clarify hierarchy of objects: global, db, schema, etc.

doc:  clarify hierarchy of objects:  global, db, schema, etc.

The previous wording was confusing because it wasn't in decreasing order
and had to backtrack.  Also clarify role/user wording.

Reported-by: jbird@nuna.com

Discussion: https://postgr.es/m/158057750885.1123.2806779262588618988@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4bd53803ef73b43124693e4a96e973e9329b8361

Modified Files
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doc/src/sgml/ddl.sgml       | 15 +++++++--------
doc/src/sgml/manage-ag.sgml | 16 +++++++---------
2 files changed, 14 insertions(+), 17 deletions(-)