[Bug] Clarifications to What's New in C# 7 and Tuples topics

[BillWagner]

@richlander suggested several updates to the What's new in C# 7 topic here That PR has a tighter focus (how to describe the dependency on .NET 4.7, or the System.ValueTuple package).

A summary of the changes requested are:

• Move the dependency on ValueTuple and ValueTask to opening of the Tuples and generalized async return types respectively.
• More clearly separate language updates from library types. (See Issue C# Guide: Explain the language dependency on framework types #1760 for more details here)
• Tighten up some of the explanations (see referenced comment above)