Added 'Type Checker APIs' section prefix to type checker docs (#320)

* Added 'Type Checker APIs' section prefix to type checker docs

* Apply suggestions from code review

Co-authored-by: Daniel Rosenwasser <[email protected]>

---------

Co-authored-by: Daniel Rosenwasser <[email protected]>

Using-the-Compiler-API.md

@@ -607,6 +607,42 @@ const result = printer.printNode(ts.EmitHint.Unspecified, makeFactorialFunction(
 console.log(result);
 ```
 
+## Type Checker APIs
+
+Programs contain a `TypeChecker` object that provides methods for retrieving and reasoning about the types of syntax tree nodes.
+Type checker APIs generally work with two types of representations alongside AST nodes:
+
+* `Symbol`s: describes how the type system views a declared entity, such as a class, function, or variable.
+* `Type`s: describes the backing type that entities may be declared as.
+  These often have a backing `Symbol` pointing to their declaration(s).
+
+For example, given a single function declaration like:
+
+```ts
+function greet() {
+  console.log("Hello, world!");
+}
+```
+
+TypeScript will create a `Symbol` in the containing scope for `greet`.
+When looking up ("resolving") an identifier with the name `greet`, that `Symbol` can be retrieved.
+This `Symbol` will contain information about how `greet` was declared, and can be used to gain information about the type of `greet`.
+
+On that note, TypeScript will also create a type describing `greet`, which is effectively the type `() => void` - a function with no parameters and a `void` return type.
+In this case, the type will be backed by the original symbol associated with `greet`.
+
+The type checker can be retrieved like `program.getTypeChecker()`.
+Commonly used type checker APIs include:
+
+* `getSymbolAtLocation(node)`: retrieves the `Symbol` associated with an AST node
+* `getTypeAtLocation(node)`: retrieves the `Type` associated with an AST node
+* `getTypeOfSymbolAtLocation(symbol, node)`: retrieves the `Type` associated with a symbol at a specific AST node
+* `typeToString(type)`: prints a type to a human-readable string
+
+> The type checker concept of `Symbol` only coincidentally has the same name as the JavaScript concept of `Symbol`.
+> JavaScript's `Symbol` is a runtime primitive that is used to create unique identifiers.
+> TypeScript's `Symbol` is unrelated to the JavaScript `Symbol` and is used to represent the type system's view of an entity.
+
 ### Using the Type Checker
 
 In this example we will walk the AST and use the checker to serialize class information.