1. Introduction

Welcome to this tutorial on API Documentation. The main goal of this tutorial is to familiarize you with the best practices for documenting your HTML and API code, thereby making your API more accessible and easier for other developers to understand and use.

By the end of this tutorial, you will learn:
- What API Documentation is and why it's important
- How to create efficient API Documentation
- Best practices for documenting your code

Prerequisites
Basic understanding of APIs and HTML is required. Familiarity with a programming language (preferably JavaScript) would be beneficial.

2. Step-by-Step Guide

API Documentation is where developers outline the functionalities, utilities, and operations of an API. It serves as a technical content deliverable, containing instructions about how to effectively use and integrate an API.

When documenting your API, consider the following:

1. Overview: Start by providing a high-level overview of what your API does.

2. Authentication: Explain how users can gain access to your API. Include information about API keys, OAuth, etc.

3. Error Messages: Clearly define what each error message means.

4. Rate Limiting: If applicable, inform users about rate limits.

5. Endpoints and Methods: List and describe your API endpoints, methods, parameters, and sample requests and responses.

6. Code Examples: Offer code examples in multiple languages.

3. Code Examples

Example 1: Documenting an API Endpoint

/**
 * @api {get} /user/:id Request User information
 * @apiName GetUser
 * @apiGroup User
 *
 * @apiParam {Number} id User's unique ID.
 *
 * @apiSuccess {String} firstname Firstname of the User.
 * @apiSuccess {String} lastname  Lastname of the User.
 */

In the above example, we are documenting an API endpoint /user/:id which is used to get a user's information. We use annotations like '@api', '@apiName', '@apiGroup', etc. to document various aspects of the endpoint.

4. Summary

In this tutorial, we learned about API Documentation, its importance, and how to write it. We also covered some best practices for documenting your API.

For further learning, you can explore tools like Swagger or Postman, which can help you to automate the process of API documentation.

5. Practice Exercises

Exercise 1: Document a 'POST' endpoint for creating a new user.

Exercise 2: Document an 'UPDATE' endpoint for modifying an existing user's information.

Exercise 3: Document a 'DELETE' endpoint for deleting a user.

Solutions:

1. The 'POST' endpoint will require parameters like firstname, lastname, email, etc.
2. The 'UPDATE' endpoint will need the id parameter to identify the user and other parameters like firstname, lastname, etc. that need to be updated.
3. The 'DELETE' endpoint will require the id parameter to identify the user who needs to be deleted.

Remember, the key to good API documentation is clarity, completeness, and conciseness. It should provide all the necessary information without any ambiguity. Happy documenting!