[SPARK-3307] [PySpark] Fix doc string of SparkContext.broadcast()

 remove invalid docs

Author: Davies Liu <davies.liu@gmail.com>

Closes #2202 from davies/keep and squashes the following commits:

aa3b44f [Davies Liu] remove invalid docs

(cherry picked from commit e248328b39f52073422a12fd0388208de41be1c7)
Signed-off-by: Josh Rosen <joshrosen@apache.org>