[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