[SPARK-8437] [DOCS] Using directory path without wildcard for filename slow...

[SPARK-8437] [DOCS] Using directory path without wildcard for filename slow for large number of files with wholeTextFiles and binaryFiles

Note that 'dir/*' can be more efficient in some Hadoop FS implementations that 'dir/'

Author: Sean Owen <sowen@cloudera.com>

Closes #7036 from srowen/SPARK-8437 and squashes the following commits:

0e813ae [Sean Owen] Note that 'dir/*' can be more efficient in some Hadoop FS implementations that 'dir/'

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -831,6 +831,8 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
    * }}}
    *
    * @note Small files are preferred, large file is also allowable, but may cause bad performance.
+   * @note On some filesystems, `.../path/*` can be a more efficient way to read all files in a directory
+   *       rather than `.../path/` or `.../path`
    *
    * @param minPartitions A suggestion value of the minimal splitting number for input data.
    */
@@ -878,9 +880,11 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
    *   (a-hdfs-path/part-nnnnn, its content)
    * }}}
    *
-   * @param minPartitions A suggestion value of the minimal splitting number for input data.
-   *
    * @note Small files are preferred; very large files may cause bad performance.
+   * @note On some filesystems, `.../path/*` can be a more efficient way to read all files in a directory
+   *       rather than `.../path/` or `.../path`
+   *
+   * @param minPartitions A suggestion value of the minimal splitting number for input data.
    */
   @Experimental
   def binaryFiles(