Fix selser in the v2 API

• It was setting env.page.dom as the document, whereas throughout the codebase that's usually the body. This produced an unnecessary diff.
• To this point, some of the tests for the v2 API are merely documenting the expectations, and not enforcing anything. Here we add a test to ensure selser is used.
• Turns on useSelser in the API test parsoidServer.
• Renames some variables in the selectiveSerializer to reflect that we're working with a body, not a doc.
• Clarified that a new data-parsoid should not be submitted. Can only be found in the original field and applied to both new and old html.

Bug: T89411
Change-Id: I91d688d7b3942b89637b15ed2ae1a54dab22bede