expose unspill config option (NVIDIA#2566)

Signed-off-by: Rong Ou <rong.ou@gmail.com>

docs/configs.md

@@ -40,6 +40,7 @@ Name | Description | Default Value
 <a name="memory.gpu.pool"></a>spark.rapids.memory.gpu.pool|Select the RMM pooling allocator to use. Valid values are "DEFAULT", "ARENA", and "NONE". With "DEFAULT", `rmm::mr::pool_memory_resource` is used; with "ARENA", `rmm::mr::arena_memory_resource` is used. If set to "NONE", pooling is disabled and RMM just passes through to CUDA memory allocation directly. Note: "ARENA" is the recommended pool allocator if CUDF is built with Per-Thread Default Stream (PTDS), as "DEFAULT" is known to be unstable (https://github.com/NVIDIA/spark-rapids/issues/1141)|ARENA
 <a name="memory.gpu.pooling.enabled"></a>spark.rapids.memory.gpu.pooling.enabled|Should RMM act as a pooling allocator for GPU memory, or should it just pass through to CUDA memory allocation directly. DEPRECATED: please use spark.rapids.memory.gpu.pool instead.|true
 <a name="memory.gpu.reserve"></a>spark.rapids.memory.gpu.reserve|The amount of GPU memory that should remain unallocated by RMM and left for system use such as memory needed for kernels, kernel launches or JIT compilation.|1073741824
+<a name="memory.gpu.unspill.enabled"></a>spark.rapids.memory.gpu.unspill.enabled|When a spilled GPU buffer is needed again, should it be unspilled, or only copied back into GPU memory temporarily. Unspilling may be useful for GPU buffers that are needed frequently, for example, broadcast variables; however, it may also increase GPU memory usage|false
 <a name="memory.host.spillStorageSize"></a>spark.rapids.memory.host.spillStorageSize|Amount of off-heap host memory to use for buffering spilled GPU data before spilling to local disk|1073741824
 <a name="memory.pinnedPool.size"></a>spark.rapids.memory.pinnedPool.size|The size of the pinned memory pool in bytes unless otherwise specified. Use 0 to disable the pool.|0
 <a name="python.concurrentPythonWorkers"></a>spark.rapids.python.concurrentPythonWorkers|Set the number of Python worker processes that can execute concurrently per GPU. Python worker processes may temporarily block when the number of concurrent Python worker processes started by the same executor exceeds this amount. Allowing too many concurrent tasks on the same GPU may lead to GPU out of memory errors. >0 means enabled, while <=0 means unlimited|0

sql-plugin/src/main/scala/com/nvidia/spark/rapids/RapidsConf.scala

@@ -357,7 +357,6 @@ object RapidsConf {
         "back into GPU memory temporarily. Unspilling may be useful for GPU buffers that are " +
         "needed frequently, for example, broadcast variables; however, it may also increase GPU " +
         "memory usage")
-      .internal()
       .booleanConf
       .createWithDefault(false)