base-files: /etc/sysctl.conf: clarity in comment

base-files: /etc/sysctl.conf: clarity in comment

Make it clear to users that they should not place a custom file
in /etc/sysctl.d/ for their values and expect it to survive a
reimage.

This change is needed since these directories (/etc/foo.d/) are
generally where such files are placed on other distros.

Signed-off-by: John Audia <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/16543
Signed-off-by: Hauke Mehrtens <[email protected]>