Add introductory section plus specification of abbreviated constructor declarations

[eernstg]

This PR adds a short introductory section about the abbreviated constructor syntax which is part of the primary constructor feature bundle, and it specifies the meaning of the abbreviated constructor declarations. For example:

class MyClass(final int i) {
  new(int j) : this(j + 1); // Rather than `MyClass(int j) ...`.
}

[munificent]

Aside from a style nit about the space before (, LGTM!

[eernstg]

I built the tools from the HEAD of SDK, and dart format suddenly does not include the space in new (); and factory ();. That's surprising because I thought we had discussed this, and the behavior of the formatter confirmed that assumption. Anyway, I removed the space and the commentary about why it was there.