Document how (lazy) type checking works

Before the whole compiler and its API is documented, could you add a few details about the type checker: * What normally forces the type checking and inference of a name? * How TSC forces all types to be checked? * What are the options for an application using the compiler API to control/force type checking? Potentially relevant wiki pages for this information: * https://github.com/microsoft/TypeScript/wiki/Architectural-Overview * https://github.com/microsoft/TypeScript/wiki/Using-the-Compiler-API * https://github.com/microsoft/TypeScript/wiki/Compiler-Internals ## Search Terms documentation, type check, laziness, architecture, compiler API