Update string to float compatibility doc[skip ci]

[thirtiseven]

Closes #10037

Casting from string to double on the GPU could sometimes return incorrect results if the string contains high precision values. In Apache Spark, the values are rounded to the nearest double, whereas the Rapids accelerator truncates the values directly.

There are some tests in the issue. It looks like not an easy fix, so I think we can update the compatibility doc first.

[andygrove]

Our behavior is still inconsistent with Spark for this case, although the behavior does seem to have changed since this documentation was written. Perhaps we need to update this documentation rather than remove it?

The test below was with Spark 3.1.1

scala> val df = Seq("1.7976931348623158E308", "123").toDF("a").repartition(2)
scala> val df2 = df.withColumn("b", col("a").cast(DataTypes.DoubleType))
scala> spark.conf.set("spark.rapids.sql.enabled", false)
scala> df2.show
+--------------------+--------------------+
|                   a|                   b|
+--------------------+--------------------+
|1.797693134862315...|1.797693134862315...|
|                 123|               123.0|
+--------------------+--------------------+

scala> spark.conf.set("spark.rapids.sql.enabled", true)

scala> df2.show
24/01/08 18:34:56 WARN GpuOverrides: 
!Exec <CollectLimitExec> cannot run on GPU because the Exec CollectLimitExec has been disabled, and is disabled by default because Collect Limit replacement can be slower on the GPU, if huge number of rows in a batch it could help by limiting the number of rows transferred from GPU to CPU. Set spark.rapids.sql.exec.CollectLimitExec to true if you wish to enable it
  @Partitioning <SinglePartition$> could run on GPU
  *Exec <ProjectExec> will run on GPU
    *Expression <Alias> cast(cast(a#4 as double) as string) AS b#30 will run on GPU
      *Expression <Cast> cast(cast(a#4 as double) as string) will run on GPU
        *Expression <Cast> cast(a#4 as double) will run on GPU
    *Exec <ShuffleExchangeExec> will run on GPU
      *Partitioning <RoundRobinPartitioning> will run on GPU
      ! <LocalTableScanExec> cannot run on GPU because GPU does not currently support the operator class org.apache.spark.sql.execution.LocalTableScanExec
        @Expression <AttributeReference> a#4 could run on GPU

+--------------------+--------+
|                   a|       b|
+--------------------+--------+
|1.797693134862315...|Infinity|
|                 123|   123.0|
+--------------------+--------+

[thirtiseven]

Updated. Thanks for your test!

[hyperbolic2346]

Seems an improvement, but I am not really up on this issue.

[thirtiseven]

build