Document design of DOM objects

Several questions about the design of libsdformat's DOM objects have come up in discussions with @EricCousineau-TRI over time, including the following:

* Lifetime of DOM elements, lifetime or relationship
* role of semantics, why not just use xsd to generate structs
* How dynamic is the DOM? If you call SetPoseRelativeTo, does that modify the embedded ElementPtr? the PoseRelativeToGraph?
* Is the DOM a read-only interface to a model parsed from XML? Or would you construct a model programmatically from DOM objects?