Update documentation on optimisers and transformers

[TomAFrench]

Some of our transformers are relatively obvious on what they do (e.g. the FallBackTransformer) but others are much less so such as the CSatTransformer, for instance:

• What is CSAT in this context? Only thing I can see that seems potentially relevant is circuit satisfiability.
• The current documentation is written as if it's an optimisation whereas from chore!: Reorganise compiler in terms of optimisers and transformers #60, it's required for the circuit to be solvable.

We should update each transformer/optimiser's documentation to reflect their new status and add a short, accessible blurb on what they're doing (range.rs is a nice example but this is a very simple optimisation).