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: /messages/by-id/160399562040.7809.7335281028960123489@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c8961de1456f77d0c6875f91958045adac5a2bb2

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