docs(middleware): complete middleware author's guide

Add comprehensive documentation for middleware development in Actix Web, including:
- Detailed explanation of middleware concepts and execution flow
- Clear description of middleware traits and their responsibilities
- Guidelines for body type handling
- Best practices for middleware development
- Error handling recommendations
- Usage scenarios and anti-patterns

actix-web/src/middleware/authors-guide.md

@@ -2,16 +2,80 @@
 
 ## What Is A Middleware?
 
+Middleware in Actix Web is a powerful mechanism that allows you to add additional behavior to request/response processing. It enables you to:
+
+- Pre-process incoming requests (e.g., path normalization, authentication)
+- Post-process outgoing responses (e.g., logging, compression)
+- Modify application state through ServiceRequest
+- Access external services (e.g., sessions, caching)
+
+Middleware is registered for each App, Scope, or Resource and executed in the reverse order of registration. This means the last registered middleware is the first to process the request.
+
 ## Middleware Traits
 
+Actix Web's middleware system is built on two main traits:
+
+1. `Transform<S, Req>`: The builder trait that creates the actual Service. It's responsible for:
+
+   - Creating new middleware instances
+   - Assembling the middleware chain
+   - Handling initialization errors
+
+2. `Service<Req>`: The trait that represents the actual middleware functionality. It:
+   - Processes requests and responses
+   - Can modify both request and response
+   - Can short-circuit request processing
+   - Must be implemented for the middleware to work
+
 ## Understanding Body Types
 
+When working with middleware, it's important to understand body types:
+
+- Middleware can work with different body types for requests and responses
+- The `MessageBody` trait is used to handle different body types
+- You can use `EitherBody` when you need to handle multiple body types
+- Be careful with body consumption - once a body is consumed, it cannot be read again
+
 ## Best Practices
 
+1. Keep middleware focused and single-purpose
+2. Handle errors appropriately and propagate them correctly
+3. Be mindful of performance impact
+4. Use appropriate body types and handle them correctly
+5. Consider middleware ordering carefully
+6. Document your middleware's behavior and requirements
+7. Test your middleware thoroughly
+
 ## Error Propagation
 
+Proper error handling is crucial in middleware:
+
+1. Always propagate errors from the inner service
+2. Use appropriate error types
+3. Handle initialization errors
+4. Consider using custom error types for specific middleware errors
+5. Document error conditions and handling
+
 ## When To (Not) Use Middleware
 
+Use middleware when you need to:
+
+- Add cross-cutting concerns
+- Modify requests/responses globally
+- Add authentication/authorization
+- Add logging or monitoring
+- Handle compression or caching
+
+Avoid middleware when:
+
+- The functionality is specific to a single route
+- The operation is better handled by a service
+- The overhead would be too high
+- The functionality can be implemented more simply
+
 ## Author's References
 
 - `EitherBody` + when is middleware appropriate: https://discord.com/channels/771444961383153695/952016890723729428
+- Actix Web Documentation: https://docs.rs/actix-web
+- Service Trait Documentation: https://docs.rs/actix-service
+- MessageBody Trait Documentation: https://docs.rs/actix-web/latest/actix_web/body/trait.MessageBody.html