[MINOR] Comment improvements in ExternalSorter.

1. Clearly specifies the contract/interactions for users of this class.
2. Minor fix in one doc to avoid ambiguity.

Author: Patrick Wendell <patrick@databricks.com>

Closes apache#5620 from pwendell/cleanup and squashes the following commits:

8d8f44f [Patrick Wendell] [Minor] Comment improvements in ExternalSorter.

core/src/main/scala/org/apache/spark/util/collection/ExternalSorter.scala

@@ -53,7 +53,18 @@ import org.apache.spark.storage.{BlockObjectWriter, BlockId}
  * probably want to pass None as the ordering to avoid extra sorting. On the other hand, if you do
  * want to do combining, having an Ordering is more efficient than not having it.
  *
- * At a high level, this class works as follows:
+ * Users interact with this class in the following way:
+ *
+ * 1. Instantiate an ExternalSorter.
+ *
+ * 2. Call insertAll() with a set of records.
+ *
+ * 3. Request an iterator() back to traverse sorted/aggregated records.
+ *     - or -
+ *    Invoke writePartitionedFile() to create a file containing sorted/aggregated outputs
+ *    that can be used in Spark's sort shuffle.
+ *
+ * At a high level, this class works internally as follows:
  *
  * - We repeatedly fill up buffers of in-memory data, using either a SizeTrackingAppendOnlyMap if
  *   we want to combine by key, or an simple SizeTrackingBuffer if we don't. Inside these buffers,
@@ -65,11 +76,11 @@ import org.apache.spark.storage.{BlockObjectWriter, BlockId}
  *   aggregation. For each file, we track how many objects were in each partition in memory, so we
  *   don't have to write out the partition ID for every element.
  *
- * - When the user requests an iterator, the spilled files are merged, along with any remaining
- *   in-memory data, using the same sort order defined above (unless both sorting and aggregation
- *   are disabled). If we need to aggregate by key, we either use a total ordering from the
- *   ordering parameter, or read the keys with the same hash code and compare them with each other
- *   for equality to merge values.
+ * - When the user requests an iterator or file output, the spilled files are merged, along with
+ *   any remaining in-memory data, using the same sort order defined above (unless both sorting
+ *   and aggregation are disabled). If we need to aggregate by key, we either use a total ordering
+ *   from the ordering parameter, or read the keys with the same hash code and compare them with
+ *   each other for equality to merge values.
  *
  * - Users are expected to call stop() at the end to delete all the intermediate files.
  *
@@ -259,8 +270,8 @@ private[spark] class ExternalSorter[K, V, C](
    * Spill our in-memory collection to a sorted file that we can merge later (normal code path).
    * We add this file into spilledFiles to find it later.
    *
-   * Alternatively, if bypassMergeSort is true, we spill to separate files for each partition.
-   * See spillToPartitionedFiles() for that code path.
+   * This should not be invoked if bypassMergeSort is true. In that case, spillToPartitionedFiles()
+   * is used to write files for each partition.
    *
    * @param collection whichever collection we're using (map or buffer)
    */