ReseedingRng: fix doc comment on reseeding behavior after UNIX fork

[LiosK]

The BlockRngCore::Results of ChaCha8Core, ChaCha12Core and ChaCha20Core have been Array64<u32> since rand_chacha v0.2.0, and they actually consume up to 63 u32 values before reseeding, but the document currently reads:

When a process is forked on UNIX, the RNGs in both the parent and child processes will be reseeded just before the next call to BlockRngCore::generate, i.e. “soon”. For ChaCha and Hc128 this is a maximum of fifteen u32 values before reseeding.

This comment used to be correct because the BlockRngCore::Results was [u32; 16] in rand_chacha v0.1.

[LiosK]

Here is a quick example demonstrating that ChaCha12Core used by ThreadRng requires 63 u32 values before reseeding after a fork call: https://gist.github.com/LiosK/ea424481fb84df162a0afaabe042eedb

[dhardy]

Thanks for the correction.

This is a problem with the code and documentation being in different crates, but I don't see a good alternative here.