pgsql: Doc: Outline REPLICATION before SUPERUSER privilege

Doc: Outline REPLICATION before SUPERUSER privilege

The following docs are updated:
- High-availaility section
- pg_basebackup
- pg_receivewal

Per the principle of least privilege, we want to encourage users to
interact with those areas using roles that have replication rights, but
superusers were mentioned first.

Author: Daniel Gustafsson
Reviewed-by: Fujii Masao, Michael Paquier
Discussion: https://postgr.es/m/ECEBD212-7101-41EB-84F3-2F356E4B6401@yesql.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c5114e42fa1ddd2c44a03339ffa436e732477397

Modified Files
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doc/src/sgml/high-availability.sgml | 14 ++++++++------
doc/src/sgml/ref/pg_basebackup.sgml |  4 ++--
doc/src/sgml/ref/pg_receivewal.sgml | 13 +++++++------
3 files changed, 17 insertions(+), 14 deletions(-)