Improve Button Kind explanation

[jcfilben]

There is some confusion about Button kind. The docs should be improved to potentially include a button kind example, clarify that kind is not a prop, include the grommet version button kind was added in instead of the month and year it was added.

See more details about this issue in this slack thread: https://grommet.slack.com/archives/C09QQEG69/p1639148987111700

[jcfilben]

Additional note: the explanation for button kind incorrectly implies that primary and secondary can't be used if button.default is not defined in the theme.