random: modernize documentation comment on get_random_bytes()

The prior text was very old and made outdated references to TCP sequence
numbers, which should use one of the integer functions instead, since
batched entropy was introduced. The current way of describing the
quality of functions is just to say that it's as good as /dev/urandom,
which now all the functions are.

Fixes: f5b98461 ("random: use chacha20 for get_random_int/long")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>