History of DOM on-event handlers

Event handler's parameters, this binding, and the return value: initial info about the return value handling, and the `this` binding

setTimeout doesn't have anything to do with DOM events; event handlers are not limited to "interactive" DOM elements

Editorial review. Minor edits.

fixed sub-title

fixed title

reworded "DOM events: addEventListener()" as it is not a thing but looks like one, added 3rd type of event for ref, adjusted affected and related text for clarification and consistency

onerror: Note that the event parameter actually contains the error message as string.

"Event handler's parameters"

remove left-over cruft from the example

reorganize the page a bit: start with setting context, move "terminology" further down and use a less confusing "on<...> event handler" to disambiguate from event listeners, split "Objects that support event handlers" into several sections