Auto merge of #7337 - wilmoz:InheritanceDocumentation, r=mbrubeck

Documentation of inheritance in type ids

Aditional documentation about this issue https://github.com/servo/servo/issues/7205

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components/script/dom/mod.rs

@@ -54,6 +54,23 @@
 //! This invariant is enforced by the lint in
 //! `plugins::lints::inheritance_integrity`.
 //!
+//! The same principle applies to typeids,
+//! the derived type enum should
+//! use one addititional type (the parent class) because sometimes the parent
+//! can be the most-derived class of an object.
+//! ```ignore
+//! pub enum EventTypeId {
+//!     UIEvent(UIEventTypeId),
+//!     //others events
+//! }
+//!
+//! pub enum UIEventTypeId {
+//!    MouseEvent,
+//!    KeyboardEvent,
+//!    UIEvent, //<- parent of MouseEvent and KeyboardEvent
+//! }
+//! ```
+//!
 //! Construction
 //! ============
 //!