[SPARK-20296][TRIVIAL][DOCS] Count distinct error message for streaming

## What changes were proposed in this pull request?
Update count distinct error message for streaming datasets/dataframes to match current behavior. These aggregations are not yet supported, regardless of whether the dataset/dataframe is aggregated.

Author: jtoka <[email protected]>

Closes #17609 from jtoka/master.

Author: jtoka

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/UnsupportedOperationChecker.scala

@@ -139,9 +139,8 @@ object UnsupportedOperationChecker {
           }
           throwErrorIf(
             child.isStreaming && distinctAggExprs.nonEmpty,
-            "Distinct aggregations are not supported on streaming DataFrames/Datasets, unless " +
-              "it is on aggregated DataFrame/Dataset in Complete output mode. Consider using " +
-              "approximate distinct aggregation (e.g. approx_count_distinct() instead of count()).")
+            "Distinct aggregations are not supported on streaming DataFrames/Datasets. Consider " +
+              "using approx_count_distinct() instead.")
 
         case _: Command =>
           throwError("Commands like CreateTable*, AlterTable*, Show* are not supported with " +