Strict Null Checking : Nullable Types

https://github.com/Microsoft/TypeScript/pull/7140

> Nullable types are just syntactic sugar for union types that include type of null/undefined.

Start by reading the linked TypeScript pull request and locating the book section covering nullable types and strict null checking. Clarify the intended explanation from that context, then update the relevant documentation so the relationship between nullable types, null, undefined, and union types is clear.

Written by the indexing model from the issue text.