pgsql: Improve documentation of data_sync_retry

Improve documentation of data_sync_retry

Reflecting an updated parameter value requires a server restart, which
was not mentioned in the documentation and in postgresql.conf.sample.

Reported-by: Thomas Poty
Discussion: https://postgr.es/m/15659-0cd812f13027a2d8@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b3a156858ab03cb5f9c55dedcaf85a6249b3ec64

Modified Files
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doc/src/sgml/config.sgml                      | 3 ++-
src/backend/utils/misc/postgresql.conf.sample | 4 +++-
2 files changed, 5 insertions(+), 2 deletions(-)