Revise features, keyboard, and states/properties for File Directory T…

…reeview Example Using Computed Properties

For issue #223, modified examples/treeview/treeview-1/treeview-1a.html:
* removed redundant information about computed level, setsize, and posinset from accessibility features section.
* Added summary of treeview terminology with link to pattern.
* Revisions for editorial consistency to keyboard table.
* Updated states/properties section header.
* Revisions for editorial consistency to states and properties table.
* Removed level, setsize, and posinset rows since they are not used in this example.