cset bake doesn't document the variables option

### What problem does your feature request solve?

Command line `cset bake` doesn't really document the variables options in its help text.

### Describe the solution you'd like

The help text, and accompanying documentation should be updated.

### Describe alternatives you've considered

Currently it does print out what variables are needed after reading a recipe, but that does not help someone learning the first time.

Start by locating the Python entry point and help text for the `cset bake` command, then find the accompanying documentation for its variables option. Review how the command currently reports required variables after reading a recipe, and update the help and documentation so users can understand the option before running a recipe.

Written by the indexing model from the issue text.