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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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/90d553dd96106ab278550e85d95a5663d77d7db7

Modified Files
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doc/src/sgml/release-9.6.sgml |  2 +-
doc/src/sgml/user-manag.sgml  | 24 ++++++++++++------------
2 files changed, 13 insertions(+), 13 deletions(-)