pgsql: Doc: prefer sysctl to /proc/sys in docs and comments.

Doc: prefer sysctl to /proc/sys in docs and comments.

sysctl is more portable than Linux's /proc/sys file tree, and
often easier to use too. That's why most of our docs refer to
sysctl when talking about how to adjust kernel parameters.
Bring the few stragglers into line.

Discussion: /messages/by-id/361175.1661187463@sss.pgh.pa.us

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d53ff6a44b32c15dd150c383ffbe4b0f5dd13037

Modified Files
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doc/src/sgml/runtime.sgml | 11 ++++++-----
src/backend/postmaster/postmaster.c | 2 +-
2 files changed, 7 insertions(+), 6 deletions(-)