[4.x] Document passing an event object to dispatch() and the 5.0 removals - #60

Summary of Changes

Since 2.0, calling Dispatcher::dispatch() without an event object, the default event objects (setEvent(), addEvent(), hasEvent(), getEvent(), removeEvent(), getEvents(), clearEvents(), countEvents()), Dispatcher::triggerEvent() and Event::stop() are deprecated, and the deprecation notices say they will be removed in 5.0. docs/overview.md still uses dispatch('onAfterSomething') without an event in "Dispatching Events" and in the "Observable classes" example, and says the default event objects will be removed "in 3.0".

This PR updates the docs only:

• "Registering Custom Events": removal version corrected to 5.0, with the replacement (create the event object and pass it to dispatch()).
• "Dispatching Events": the example without an event object is gone; the text says to always pass an event object with a matching name and lists the deprecated calls and their replacements.
• "Observable classes": the example now passes new Event(...) to dispatch().

The removal itself is a separate 5.x PR.

Testing Instructions

Documentation only; review the text.

Documentation Changes Required

This is the documentation change (docs/overview.md).

Related to #50 (also edits docs/overview.md, in a different section).

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