Streaming doc : do you mean inadvertently?

Author: CrazyJvm <crazyjvm@gmail.com>

Closes #3620 from CrazyJvm/streaming-foreachRDD and squashes the following commits:

b72886b [CrazyJvm] do you mean inadvertently?

(cherry picked from commit 6eb1b6f6204ea3c8083af3fb9cd990d9f3dac89d)
Signed-off-by: Reynold Xin <rxin@databricks.com>