pgsql: Fix some documentation in pg_rewind

Fix some documentation in pg_rewind

A 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_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4b482e94a006ee3ed6df09aba8c526cdcee84496

Modified Files
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doc/src/sgml/ref/pg_rewind.sgml | 9 +++++++++
1 file changed, 9 insertions(+)