Add details of how `val`/`var` properties work w.r.t. inheritance and overriding

[ice-phoenix]

interface I1 {
    var ordinal: Int
}

interface I2 {
    val ordinal: Int
}

class Case1(override var ordinal: Int) : I2, I1

// ERROR!
class Case2(override val ordinal: Int) : I2, I1 // VAR_OVERRIDDEN_BY_VAL

[ice-phoenix]

Most important details done in #59