Clarify documentation on randomizer reuse

[hdevalence]

The docs for randomize say

You MUST NOT reuse the randomized key package for more than one signing.

https://github.com/ZcashFoundation/frost/blob/main/frost-rerandomized/src/lib.rs#L55

It would be helpful to clarify what the impact of randomizer reuse is -- does reusing the randomizer just result in linkability between signatures, or does it compromise key material?