## What changes were proposed in this pull request?
default value for spark.sql.broadcastTimeout is 300s. and this property do not show in any docs of spark. so add "spark.sql.broadcastTimeout" into docs/sql-programming-guide.md to help people to how to fix this timeout error when it happenned

## How was this patch tested?

not need

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

…ide.md

JIRA_ID:SPARK-16870
Description:default value for spark.sql.broadcastTimeout is 300s. and this property do not show in any docs of spark. so add "spark.sql.broadcastTimeout" into docs/sql-programming-guide.md to help people to how to fix this timeout error when it happenned
Test:done

Author: keliang <keliang@cmss.chinamobile.com>

Closes #14477 from biglobster/keliang.