pgsql: docs: change "default role" wording to "predefined role"

docs: change "default role" wording to "predefined role"

The new wording was determined to be more accurate.  Also, update
release note links that reference these sections.

Reported-by: rirans@comcast.net

Discussion: https://postgr.es/m/157742545062.1149.11052653770497832538@wrigleys.postgresql.org

Backpatch-through: 9.6

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/920c6add70c803ae1dcaab9358330320a60de470

Modified Files
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doc/src/sgml/user-manag.sgml | 28 ++++++++++++++--------------
1 file changed, 14 insertions(+), 14 deletions(-)