Specs: clean up the last remaining mentions of documents

sky/specs/style.md

 Sky Style Language
 ==================
 
 Planed changes
 --------------
 
 Add //-to-end-of-line comments to be consistent with the script
 language.
 
 
@@ skipping 141 matching lines @@
 
 ```css
 tagname[attrname]#id:host.class.class[attrname=value]::foo
 ```
 
 In debug mode, giving two IDs, or the same selector twice (e.g. the
 same classname), or specifying other redundant or conflicting
 selectors (e.g. [foo][foo=bar], or [foo=bar][foo=baz]) will be
 flagged.
 
-Alternatively, a selector can be the special value "@document",
+Alternatively, a selector can be the special value "@root",
 optionally followed by a pseudo-element, as in:
 
 ```css
-@document::bar
+@root::bar
 ```
 
 
 Value Parser
 ------------
 
 ```javascript
 class StyleToken {
   constructor (String king, String value);
   readonly attribute String kind;
@@ skipping 389 matching lines @@
 if a Rule has started applying, or a Rule stopped applying, to an
 element, dart:sky calls thatElement.style.clearFrameStyles() and then,
 for each Rule that now applies, calls
 thatElement.style.addFrameStyles() with the relevant StyleDeclaration
 and pseudoElement from each such Rule.
 
 
 Update the render tree
 ----------------------
 
-Simultaneously walk the tree rooted at the application Document,
-taking into account shadow trees and child distribution, and the tree
-rooted at the document's RenderNode.
+Simultaneously walk the tree rooted at the application's element
+tree's root node, taking into account shadow trees and child
+distribution, and the tree rooted at that Root node's RenderNode.
 
 If you come across a node that doesn't have an assigned RenderNode,
 then create one, placing it in the appropriate place in the RenderTree
 tree, after any nodes marked isGhost=true, with ownerLayoutManager
 pointing to the parent RenderNode's layoutManager, if it has one, and,
 if it has one and autoreap is false on that layout manager, mark the
 new node "isNew". (This means that when a node is marked isNew, the
 layout manager has already laid out at least one frame.)
 
 For each element, if the node's needsManager is true, call
@@ skipping 473 matching lines @@
 ```
 
 The other elements don't have any default styles.
 
 These declarations are all shared between all the elements (so e.g. if
 you reach in and change the declaration that was added to a ``span``
 element, you're going to change the styles of all the other
 default-hidden elements). (In other words, in the code snippets above,
 the ``d`` variable is initialised in shared code, and only the
 addStyles() call is per-element.)