Address Early Adpoter's feedback

This PR addresses all of the points in the feedback as discussed in #8, for @honzajavorek to review & merge
^^

- in Value Member's `description` property description, there's `member type` mentioned
- in Mixin's description there's `member type` mentioned
- in One Of block there are `member types` mentioned three times

(can't comment directly there as it's unchanged area)