Extract event handlers chain documentation in a separate section.

Extract event handlers chain documentation in a separate section.

The explanation of event handlers chaining was too big and distracted from the
main point of the event processing section which was to explain in which order
different handlers are looked up.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61746 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775