IncludeTree docs improvements

[ascott18]

• Remove section about out parameters, or at least remove most of the content and add a deprecation notice. Attaching an IncludeTree to an ItemResult is the only path that should be used going forward.
• Bit about "you must use IncludedSeparately" isn't correct anymore - this isn't necessary anymore and normal .Include can always be used (I'm pretty sure, anyway).
• Bit about Enumerable.Empty<MyNonDbClass is outdated - there's static methods on IncludeTree that make this easier now.