Correct documentation on swift container names.

Null characters are no longer allowed (since grizzly)
https://github.com/openstack/swift/blob/master/swift/common/constraints.py#L203

Max container name length is 256, inclusive.
https://github.com/openstack/swift/blob/master/swift/proxy/controllers/container.py#L122

Change-Id: Idf52f730fc1c83d8e1cee11676ee0c6e797cc4fb
Merge "Correct documentation on swift container names."