HostedFiles: Missing API documentation

[Bryce Harrington]

The docs at https://help.launchpad.net/API/launchpadlib has a section about Hosted Files with some examples, however most of this is not documented in the API documentation itself at https://edge.launchpad.net/+apidoc/.

The bug_attachment object is here: https://edge.launchpad.net/+apidoc/#bug_attachment

But still needed is docs for the attachment content (data_link), since there is some important attachment info (filename, content-type, length) which can only be obtained via this object.