[SPARK-4481][Streaming][Doc] Fix the wrong description of updateFunc (backport for branch-1.2)

backport for branch-1.2 as per #3356

Author: zsxwing <zsxwing@gmail.com>

Closes #3376 from zsxwing/SPARK-4481-branch-1.2 and squashes the following commits:

53b94e8 [zsxwing] Fix the wrong description of updateFunc