Update Callback.rst

add square brackets to yaml codeblock (constraints: - Callback: [validate]). Otherwise an error "Expected argument of type array, string given" is thrown
[Components][HttpKernel] Add FINISH_REQUEST to events table
Merge pull request #3271 from berny46/patch-1

Update Callback.rst
Merge pull request #3274 from bicpi/document_finish_request_event

[Components][HttpKernel] Add FINISH_REQUEST to events overview table
adding XML code block