Clarify the document element behavior.

pugixml currently unconditionally accepts documents with multiple
top-level element nodes in absence of parse_fragment. This is an
unfortunate omission; while it can be corrected, it will result in
regressions for some users, and it's trivial to perform the validity
check after the parse is done.

Because of this, for now we're just going to amend documentation here to
both highlight this in the W3C Conformance section, but also to more
strongly push users into realizing that there's just a single document
element (normally).

We might decide to change the behavior here to prohibit such documents
by default in the future, but for now a documentation change seems like
a better tradeoff.

Fixes #337