Add an example for the variant generator and specify FQCNs used

- Provide an explicit example, with details of options and variants generated.
- Improve descriptions.
- Add use statement for `VariantGenerator` to code block.
- Define FQCN for `VariantGeneratorInterface`
Correct variable name
Merge pull request #230 from adamelso/patch-3

Add an example for the variant generator and specify FQCNs used