Improve demo dialect documentation

[Technologicat]

We should provide short documentation of the additional builtins in each demo dialect directly in their READMEs, preferably with usage examples.

Also, link to the relevant section of the unpythonic docs for each construct.

[Technologicat]

Might be fixed in unpythonic later now that the dialects live there, but the copy of the docs here won't be updated further.