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
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1bb41c6ad795e018e396eb3e216b2d21fadc865a

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