Fix #144: editorial for several sections.

[yongsheng]

1. Refer HTML51 instead of old versions.
2. Return the empty string otherwise for 'ordered set serializer'.
3. Remove quotes for kinds of DOM exceptions & errors.
4. Change the description of stringifier steps of Range to 'stringification behavior' for linking.
5. Use 'current global object' and 'window's associated document' from HTML5.1.
6. Remove the 9th step of 'split a Text node' for it's has been done in 'Replace data with node'.
7. Correct the definition of Element's qualified name and their references.
8. Add the 'context object' for attributes' getter.
9. Bring 'throw errors' for ChildNode.replaceWith() method.
10. Correct the references of attributes of MutationObserverInit.