Document and fix DOMSourceAnydata streaming
W3C DOM requires each document to have a single root element, and
we are using the anydata element as the container. This is required
because anydata (and anyxml) can hold a list, in which case we would
be faced with a document which has multiple root elements.
Document this fact and adjust streaming interfaces/handling to take
this into account.
JIRA: YANGTOOLS-993
Change-Id: Ifcc7777d60359c942b8f033307ab63295525ca1e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>