pgsql: 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: https://postgr.es/m/CABUevEz5bpvbwVsYCaSMV80CBZ5-82nkMzbb+Bu=h1m=rLdn=g@mail.gmail.com
Backpatch-through: 9.5

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/064b3fcbdb198f04ee74e01dd48b3d75448b3c4c

Modified Files
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doc/src/sgml/ref/pg_rewind.sgml | 34 ++++++++++++++++++++++++++++++----
1 file changed, 30 insertions(+), 4 deletions(-)