also discourage the usage of notToEqual in case of nullable BigDecimal

[robstoll]

*Api": all
Platform (all, jvm, js): all
Extension (none, kotlin 1.3): none

Code related feature

// guides the user to use toEqualNumerically or toEqualIncludingScale
expect(BigDecimal("10")).toEqual(BigDecimal("10.0")) 
expect(BigDecimal("10") as BigDecimal?).toEqual(BigDecimal("10.0"))

// guides the user to use notToEqualNumerically or notToEqualIncludingScale
expect(BigDecimal("10")).notToEqual(BigDecimal("10.0")) 

// does not guide the user
expect(BigDecimal("10") as BigDecimal?).toEqual(BigDecimal("10.0"))