Object versioning docs only discuss versioned_writes

[Tim Burke]

We have a page discussing object versioning at https://docs.openstack.org/swift/latest/api/object_versioning.html -- but it only talks about versioning with X-Versions-Location and X-History-Location.

We should also discuss the more-S3-like object_versioning method of versioning. In fact, we probably ought to lead with it, given that https://docs.openstack.org/swift/latest/middleware.html#module-swift.common.middleware.versioned_writes.object_versioning describes the other two as "legacy modes".