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: https://postgr.es/m/20200817165222.GA31806@momjian.us
Backpatch-through: 9.5
Branch
------
REL_10_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/f6679e0b4b435d2f9977bf0d03d10cf6c908c3e1
Modified Files
--------------
doc/src/sgml/config.sgml | 2 +-
doc/src/sgml/monitoring.sgml | 46 ++++++++++++++++++++++----------------------
2 files changed, 24 insertions(+), 24 deletions(-)