Variances: replace lowerBoundStack with a single flag
Variances: replace lowerBoundStack with a single flag

Add documentation to the two different variance validation methods:
`validateDefinition` and `validateVarianceOfPolyTypesIn`.

scala/scala,lrytz/scala,scala/scala,lrytz/scala,scala/scala,lrytz/scala,scala/scala,lrytz/scala,scala/scala,lrytz/scala,lrytz/scala,scala/scala