layout: Document the root flow.

src/servo/layout/flow.rs

@@ -14,27 +14,29 @@
 ///
 /// Currently, the important types of flows are:
 /// 
-/// * `BlockFlow`: a flow that establishes a block context. It has several child flows, each of
+/// * `BlockFlow`: A flow that establishes a block context. It has several child flows, each of
 ///   which are positioned according to block formatting context rules (CSS block boxes). Block
 ///   flows also contain a single `GenericBox` to represent their rendered borders, padding, etc.
 ///   (In the future, this render box may be folded into `BlockFlow` to save space.)
 ///
-/// * `InlineFlow`: a flow that establishes an inline context. It has a flat list of child
+/// * `InlineFlow`: A flow that establishes an inline context. It has a flat list of child
 ///   boxes/flows that are subject to inline layout and line breaking and structs to represent
 ///   line breaks and mapping to CSS boxes, for the purpose of handling `getClientRects()` and
 ///   similar methods.
+///
+/// * `RootFlow`: The flow at the root of the tree. This flow behaves like a `BlockFlow`, except
+///   that stretches to the boundaries of the viewport.
 
 use dom::node::AbstractNode;
 use layout::block::{BlockFlowData, BlockLayout};
 use layout::box::RenderBox;
 use layout::context::LayoutContext;
 use layout::debug::DebugMethods;
 use layout::display_list_builder::DisplayListBuilder;
-use layout::inline::{InlineFlowData, InlineLayout};
-use layout::root::{RootFlowData, RootLayout};
+use layout::inline::{InlineFlowData};
+use layout::root::{RootFlowData};
 
 use core::cell::Cell;
-use core::ptr;
 use geom::point::Point2D;
 use geom::rect::Rect;
 use gfx::display_list::DisplayList;