Document download call blocking and non-blocking calls

[dreamflasher]

From the documentation it's not clear what the current behaviour of dl.download() is. But either way it would be great if you would add an option to toggle the behaviour: blocking or non-blocking.

[dreamflasher]

Apparently this does exist, but is undocumented:

blocking:

parfive/parfive/downloader.py

Line 315 in 54d81fa

def download(self):

Downloader.download()

non-blocking:

parfive/parfive/downloader.py

Line 257 in 54d81fa

async def run_download(self):

Downloader.run_download()

I suggest do add this to the documentation.