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

src/backend/postmaster/postmaster.c

@@ -4909,7 +4909,7 @@ SubPostmasterMain(int argc, char *argv[])
 	 * If testing EXEC_BACKEND on Linux, you should run this as root before
 	 * starting the postmaster:
 	 *
-	 * echo 0 >/proc/sys/kernel/randomize_va_space
+	 * sysctl -w kernel.randomize_va_space=0
 	 *
 	 * This prevents using randomized stack and code addresses that cause the
 	 * child process's memory map to be different from the parent's, making it