[SPARK-24324][PYTHON][FOLLOWUP] Grouped Map positional conf should have deprecation note

## What changes were proposed in this pull request?

Followup to the discussion of the added conf in SPARK-24324 which allows assignment by column position only.  This conf is to preserve old behavior and will be removed in future releases, so it should have a note to indicate that.

## How was this patch tested?

NA

Author: Bryan Cutler <[email protected]>

Closes #21637 from BryanCutler/arrow-groupedMap-conf-deprecate-followup-SPARK-24324.

Author: BryanCutler

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -1167,7 +1167,7 @@ object SQLConf {
       .doc("When true, a grouped map Pandas UDF will assign columns from the returned " +
         "Pandas DataFrame based on position, regardless of column label type. When false, " +
         "columns will be looked up by name if labeled with a string and fallback to use " +
-        "position if not.")
+        "position if not. This configuration will be deprecated in future releases.")
       .booleanConf
       .createWithDefault(false)