Document discriminated unions
Document discriminated unions

Also move intersection types together with union types.
Also some minor fixups.
Squashed commit of the following:

commit a86ccc5d9170c7581a7cf63c8d60301ea450b96f
Merge: 620350d 819891d
Author: Nathan Shively-Sanders <nathansa@microsoft.com>
Date:   Mon Jun 27 10:56:02 2016 -0700

    Merge branch 'release-2.0' into discriminated-unions-WIP

commit 620350dcfc0fe0be8bed7dd6894f7436e45dc059
Author: Nathan Shively-Sanders <nathansa@microsoft.com>
Date:   Mon Jun 27 10:49:47 2016 -0700

    Finish exhaustiveness checking

commit e5fabc82df929a04cf0f695d8f5f7aead19106b3
Author: Nathan Shively-Sanders <nathansa@microsoft.com>
Date:   Mon Jun 27 10:04:01 2016 -0700

    Mostly done

commit 572e93a1a9bc956011e12add5c60c18de1712b57
Author: Nathan Shively-Sanders <nathansa@microsoft.com>
Date:   Mon Jun 27 09:31:56 2016 -0700

    One new example and minor wording tweaks

commit 87f684499ce7c72a1d2067dd90f2889ecd164ea3
Author: Nathan Shively-Sanders <nathansa@microsoft.com>
Date:   Fri Jun 24 16:09:49 2016 -0700

    Preparatory reorganisation

    1. Move intersection types to the beginning.
    2. Improve wording a little.

Microsoft/TypeScript-Handbook