Revise features, keyboard, and states/properties for Navigation Treev…

…iew Example Using Declared Properties

For issue #226, modified examples/treeview/treeview-2/treeview-2b.html:
* Under accessibility features, removed redundant information about computed level, setsize, and posinset.
* Under accessibility features, removed incorrect statement: "The expandable <code>treeitem</code> widgets cannot behave as links, their only action can be to open and close a leaf in the tree."
* Added section summarizing treeview terminology with link to the pattern.
* Corrected action defined for enter and space (had the action from the file tree example).
* Revisions for editorial consistency to keyboard table.
* Updated states/properties section header.
* For the tree element, corrected row for label. It specified aria-label instead of aria-labelledby.
* Revisions for editorial consistency to states and properties table.