Adjust documentation for syncfs().

Adjust documentation for syncfs().

Commit 8c16ad3b43 created a new appendix for syncfs(), which is
excessive for such a small amount of content.  This commit moves
the description of the caveats to be aware of when using syncfs()
back to the documentation for recovery_init_sync_method.  The
documentation for the other utilities with syncfs() support now
directs readers to recovery_init_sync_method for information about
these caveats.

Reported-by: Peter Eisentraut, Robert Haas
Suggested-by: Robert Haas
Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/42804669-7063-1320-ed37-3226d5f1067d%40eisentraut.org
Discussion: https://postgr.es/m/CA%2BTgmobUiqKr%2BZMCLc5Qap-sXBnjfGUU%2BZBmzYEjUuWyjsGr1g%40mail.gmail.com