docs: sysctl/kernel: document random

See Documentation/filesystems/devpts.rst.

random

======

This is a directory, with the following entries:

* ``boot_id``: a UUID generated the first time this is retrieved, and

  unvarying after that;

* ``entropy_avail``: the pool's entropy count, in bits;

* ``poolsize``: the entropy pool size, in bits;

* ``urandom_min_reseed_secs``: obsolete (used to determine the minimum

  number of seconds between urandom pool reseeding).

* ``uuid``: a UUID generated every time this is retrieved (this can

  thus be used to generate UUIDs at will);

* ``write_wakeup_threshold``: when the entropy count drops below this

  (as a number of bits), processes waiting to write to ``/dev/random``

  are woken up.

If ``drivers/char/random.c`` is built with ``ADD_INTERRUPT_BENCH``

defined, these additional entries are present:

* ``add_interrupt_avg_cycles``: the average number of cycles between

  interrupts used to feed the pool;

* ``add_interrupt_avg_deviation``: the standard deviation seen on the

  number of cycles between interrupts used to feed the pool.

randomize_va_space

==================