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