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