Enhanced example for memory usage executor, sync for all docs

It seems, html help already had one free instance with -s option for persistance,
though still used inner non-persistant chain for humanized output.

doc/tint2.md

@@ -694,10 +694,11 @@ execp_markup = 1
 ##### Memory usage
 
 ```
+# "stdbuf -oL" noticed to give better performance than fflush() from awk code
 execp = new
-execp_command = free | awk '/^-/ { printf "Mem: '$(free -h | awk '/^Mem:/ { print $2 }')' %.0f%%\n", 100*$3/($3+$4); fflush(stdout) }'
-execp_interval = 5
-execp_continuous = 0
+execp_command = free -b -s1 | stdbuf -oL awk '/^Mem:/ { printf "Mem: %.1fGi %.0f%%\n", $2 / (2^30), 100 * ($2 - $7) / $2 }'
+execp_interval = 1
+execp_continuous = 1
 ```
 
 ##### Network load