[Spark 2060][SQL] Querying JSON Datasets with SQL and DSL in Spark SQL

JIRA: https://issues.apache.org/jira/browse/SPARK-2060
Maybe more clear if you document it this way:
```
Widening types might result in loss of precision the following cases:
 - IntegerType to FloatType
 - LongType to FloatType
 - LongType to DoubleType
```