Обсуждение: pgsql: doc: update bgwriter description

doc:  update bgwriter description

This clarifies exactly what the bgwriter does, which should help with
tuning.

Reported-by: Chris Wilson

Discussion: https://postgr.es/m/160399562040.7809.7335281028960123489@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bea246117b402e59340c9efa0bdbdbe7c1ea8554

Modified Files
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doc/src/sgml/config.sgml | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)