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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master

Details
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https://git.postgresql.org/pg/commitdiff/4ee6740167b6a311660a1e8752447a496dd0d235

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(-)