random: re-add removed comment about get_random_{u32,u64} reseeding

The comment about get_random_{u32,u64}() not invoking reseeding got
added in an unrelated commit, that then was recently reverted by
0313bc27

 ("Revert "random: block in /dev/urandom""). So this adds
that little comment snippet back, and improves the wording a bit too.

Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>