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: https://postgr.es/m/361175.1661187463@sss.pgh.pa.us

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a0d87e2a92ea37d136b5ce69af2e8cd7c6127a35

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(-)