Fix some documentation in pg_rewind
Fix some documentation in pg_rewind

Since 11, it is possible to use a non-superuser role when using an
online source cluster with pg_rewind as long as the role has proper
permissions to execute on the source all the functions used by
pg_rewind, and the documentation stated that a superuser is necessary.
Let's add at the same time all the details needed to create such a
role.

A second confusion which comes a lot from users is that it is necessary
to issue a checkpoint on a freshly-promoted standby so as its control
file has up-to-date timeline information which is used by pg_rewind to
validate the operation.  Let's document that properly.  This is
back-patched down to 9.5 where pg_rewind has been introduced.

Author: Michael Paquier
Reviewed-by: Magnus Hagander
Discussion: 022580a91d74ebdf7eaef302e79d30f131095bed@mail.gmail.com
Backpatch-through: 9.5

greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb