[docs] Document Determinism Properties

**Is your feature request related to a problem? Please describe.**
I've just been asked for the nth time whether zstd is deterministic and launched into the same spiel about "yes, but in a constrained way, and why are you asking??". This is a recurring siren call that our users want, and we should write up a description of the properties Zstd does and doesn't provide / an FAQ / guidance.

**Describe the solution you'd like**
We should write a doc we can link people to.

**Describe alternatives you've considered**
We could not do it.