docs: sysctl/kernel: document random

This documents the random directory, based on the behaviour seen in
drivers/char/random.c.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20200623112514.10650-1-steve@sk2.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>