Explain the role of Deterministics in the docs

[ricardoV94]

Description

While #6695 is a bit dramatic and I have changed my mid about it a couple of times, we should at least mention on the docstrings of Deterministic that they are optional.

We could show explicitly the same code with and without them.

Also emphasize when they're needed:

• You want to easily access values in the trace
• You want to target for model transforms like do/observe