add docs

Author: jose-torres

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/continuous/ContinuousDataSourceRDDIter.scala

@@ -52,6 +52,7 @@ class ContinuousDataSourceRDD(
   }
 
   override def compute(split: Partition, context: TaskContext): Iterator[UnsafeRow] = {
+    // If attempt number isn't 0, this is a task retry, which we don't support.
     if (context.attemptNumber() != 0) {
       throw new ContinuousTaskRetryException()
     }

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/continuous/ContinuousTaskRetryException.scala

@@ -19,5 +19,8 @@ package org.apache.spark.sql.execution.streaming.continuous
 
 import org.apache.spark.SparkException
 
+/**
+ * An exception thrown when a continuous processing task runs with a nonzero attempt ID.
+ */
 class ContinuousTaskRetryException
   extends SparkException("Continuous execution does not support task retry", null)