[SPARK-15894][SQL][DOC] Update docs for controlling #partitions

## What changes were proposed in this pull request?
Update docs for two parameters `spark.sql.files.maxPartitionBytes` and `spark.sql.files.openCostInBytes ` in Other Configuration Options.

## How was this patch tested?
N/A

Author: Takeshi YAMAMURO <linguin.m.s@gmail.com>

Closes #13797 from maropu/SPARK-15894-2.

docs/sql-programming-guide.md

@@ -2015,6 +2015,23 @@ that these options will be deprecated in future release as more optimizations ar
 
 <table class="table">
   <tr><th>Property Name</th><th>Default</th><th>Meaning</th></tr>
+  <tr>
+    <td><code>spark.sql.files.maxPartitionBytes</code></td>
+    <td>134217728 (128 MB)</td>
+    <td>
+      The maximum number of bytes to pack into a single partition when reading files.
+    </td>
+  </tr>
+  <tr>
+    <td><code>spark.sql.files.openCostInBytes</code></td>
+    <td>4194304 (4 MB)</td>
+    <td>
+      The estimated cost to open a file, measured by the number of bytes could be scanned in the same
+      time. This is used when putting multiple files into a partition. It is better to over estimated,
+      then the partitions with small files will be faster than partitions with bigger files (which is
+      scheduled first).
+    </td>
+  </tr>
   <tr>
     <td><code>spark.sql.autoBroadcastJoinThreshold</code></td>
     <td>10485760 (10 MB)</td>