pgsql: doc: improve docs so config value default units are clearer

doc:  improve docs so config value default units are clearer

Previously, our docs would say "Specifies the number of milliseconds"
but it wasn't clear that "milliseconds" was merely the default unit.
New text says "Specifies duration (defaults to milliseconds)", which is
clearer.

Reported-by: basil.bourque@gmail.com

Discussion: https://postgr.es/m/15912-2e35e9026f61230b@postgresql.org

Backpatch-through: 12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6816497cd78a9b498785fa54af7ba0a41051ad24

Modified Files
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doc/src/sgml/config.sgml | 73 ++++++++++++++++++++++++++----------------------
1 file changed, 39 insertions(+), 34 deletions(-)