Update syntax.md

Some inconsistencies pointed out in FB comments
Introduce a supported way to slice children

Introduces a counterpart to mapChildren. It excludes empty children just as
mapChildren for compatibility. With might introduce something like
sliceChildrenIncludingEmptyValues at some point.
Injectable DOMProperty configs, and add back ID attribute

https://github.com/facebook/react/pull/141
Fix ChangeEventPlugin for IE8 and IE9

This fixes a bug with `ChangeEventPlugin` in IE8 and IE9. The extend of this bug includes:

 - On IE8, not firing `onChange` immediately after `value` of an input is changed.
 - On IE9, not firing `onChange` when backspacing.
Faster Listener Deletion

Whenever a component is unmounted, we delete all listeners that might have been attached. This sucks because most applications, Facebook included, do not use every listener. There's a lot of wasted computation, especially if many components are mounted and unmounted.

This changes `deleteAllListeners` to more delete listeners more efficiently.
Inject `ReactDOMInput`

Changes `<input>` in React to use `ReactDOMInput` which supports `value` and `defaultValue` instead of the current, arguably broken `value` behavior.
Add finally block for clearing ReactUpdates state

If any component throws during reconciliation any subsequent reconciliation will break badly because ReactUpdates will be in an inconsistent state.
Add a finally block to prevent this.
Add `hidden` attribute to DOM properties.