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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/384199ec505955ca1777b30f1ceea4b33ac82f4b

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(-)