Clarify documentation on retrieving md5 and/or crc32c hash for blobs without downloading

[tcwalther]

Update: Just briefly after posting the issue, I found out how to do it. I'm thus recommending to update the documentation to clarify how to do this.

If I understand it correctly, retrieving a blob's md5 or crc32c hash without downloading it requires calling reload():

>>> blob = bucket.blob("gs://some/url")
>>> blob.crc32c
None
>>> blob.reload()
>>> blob.crc32c
'quMJjg=='

It took me almost an hour to find that out, including browsing the documentation, browsing SO, cloning the source and having a look around there. Eventually I found this SO post which implied that blob.crc32c actually works, and then using tab-completion trial-and-error in ipython I found the reload() method.

I think it would be great if the documentation clarified this :-).

Use Case

Checking whether a remote file needs to be downloaded when a local file of the same filename already exists.