Document rules for implicit type conversions

Although there is some documentation about implicit type conversions that are allowed in Hive there is no specification defining how conversions should work when the types are not the same (which type should be converted to which type).

This makes query results hard to interpret and may lead to clients complaining about wrong results even when the behavior is expected. Moreover, the absence of a specification makes potential fixes harder to land since it is difficult to agree on what is the expected behavior.

It would really help if there was a document such as the one provided by SQLServer clearly stating what should be the result of a conversion between two types.