pgsql: docs: replace "stable storage" with "durable" in descriptions

docs: replace "stable storage" with "durable" in descriptions

For PG, "durable storage" has a clear meaning, while "stable storage"
does not, so use the former.

Discussion: /messages/by-id/20200817165222.GA31806@momjian.us

Backpatch-through: 9.5

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0d447ba962bbaa40f9d47d563086f1debf562b20

Modified Files
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doc/src/sgml/config.sgml | 2 +-
doc/src/sgml/monitoring.sgml | 48 ++++++++++++++++++++++----------------------
2 files changed, 25 insertions(+), 25 deletions(-)