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