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
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/384497f34de5435921ed2c32d29874f4f7532c3a

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