doc: add a notice about memory accounting overhead
doc: add a notice about memory accounting overhead

The documentation for Debian and Ubuntu explains how to enable memory
and swap accounting but doesn't explain why it is disabled in the first
place. The problem with those subsystems is that they incurs a
performance hit even when not used at all. Add this explanation.

The provided figure are quite vague. The memory overhead is easily
verifiable. It is for example cited in [RedHat documentation][]. For the
performance hit, maybe the performance is better now, but a few years
ago, it was said to be [between 6 and 15%][].

The goal is that people don't just enable memory accounting if they
don't have a use for it.

[RedHat documentation]: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Resource_Management_Guide/sec-memory.html
[between 6 and 15%]: https://lwn.net/Articles/517562/

Signed-off-by: Vincent Bernat <87e1f221a672a14a323e57bb65eaea19d3ed3804@bernat.im>
(cherry picked from commit 56b33e9f27fa9dc50cec16392ca01bd4234390f9)

jamesodhunt/docker,clearlinux/docker,clearlinux/docker,jamesodhunt/docker,clearlinux/docker,jamesodhunt/docker