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