[SPARK-9535][SQL][DOCS] Modify document for codegen.

#7142 made codegen enabled by default so let's modify the corresponding documents.

Closes #7142

Author: KaiXinXiaoLei <huleilei1@huawei.com>
Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #7863 from sarutak/SPARK-9535 and squashes the following commits:

0884424 [Kousuke Saruta] Removed a line which mentioned about the effect of codegen enabled
3c11af0 [Kousuke Saruta] Merge branch 'sqlconfig' of https://github.com/KaiXinXiaoLei/spark into SPARK-9535
4ee531d [KaiXinXiaoLei] delete space
4cfd11d [KaiXinXiaoLei] change spark.sql.planner.externalSort
d624cf8 [KaiXinXiaoLei] sql config is wrong

docs/sql-programming-guide.md

@@ -1885,11 +1885,10 @@ that these options will be deprecated in future release as more optimizations ar
   </tr>
   <tr>
     <td><code>spark.sql.codegen</code></td>
-    <td>false</td>
+    <td>true</td>
     <td>
       When true, code will be dynamically generated at runtime for expression evaluation in a specific
-      query.  For some queries with complicated expression this option can lead to significant speed-ups.
-      However, for simple queries this can actually slow down query execution.
+      query. For some queries with complicated expression this option can lead to significant speed-ups.
     </td>
   </tr>
   <tr>
@@ -1901,7 +1900,7 @@ that these options will be deprecated in future release as more optimizations ar
   </tr>
   <tr>
     <td><code>spark.sql.planner.externalSort</code></td>
-    <td>false</td>
+    <td>true</td>
     <td>
       When true, performs sorts spilling to disk as needed otherwise sort each partition in memory.
     </td>