pgsql: Doc: clarify behavior of ALTER DEFAULT PRIVILEGES ... IN SCHEMA.

Doc: clarify behavior of ALTER DEFAULT PRIVILEGES ... IN SCHEMA.

The existing text stated that "Default privileges that are specified
per-schema are added to whatever the global default privileges are for
the particular object type".  However, that bare-bones observation is
not quite clear enough, as demonstrated by the complaint in bug #16124.
Flesh it out by stating explicitly that you can't revoke built-in
default privileges this way, and by providing an example to drive
the point home.

Back-patch to all supported branches, since it's been like this
from the beginning.

Discussion: https://postgr.es/m/16124-423d8ee4358421bc@postgresql.org

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e5df9bb0179512bef6d9e1b51b3d07ca4db3f4ec

Modified Files
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doc/src/sgml/ref/alter_default_privileges.sgml | 28 ++++++++++++++++++++------
1 file changed, 22 insertions(+), 6 deletions(-)