Mixin reference should describe how mixins are defined

[VKobeliatsky]

Currently, looking at mixins reference page one cannot find how are they defined and parametrized (mixins with parethesis). Variants page gets this right, both usage and definition are described well. Would be nice to have the same structure for mixins page, too.

[idoros]

I hope javascript mixins will be a last resort kinda-thing. Most content of variants should actually be part of the mixins page, and There should be a guide explaining javascript mixins and when to resort to them.

[tomrav]

Resolved in https://stylable.io/docs/references/mixins#javascript-mixins If you find anything missing, let us know.