chromeos: Cleanup comments and DEPS in //ash

ash/README.md

+Ash
+---
+Ash is the "Aura Shell", the window manager and system UI for Chrome OS.
+Ash uses the views UI toolkit (e.g. views::View, views::Widget, etc.) backed
+by the aura native widget and layer implementations.
+
+Ash sits below chrome in the dependency graph (i.e. it cannot depend on code
+in //chrome). It has a few dependencies on //content, but these are isolated
+in their own module in //ash/content. This allows targets like ash_unittests
+to build more quickly.
+
+Tests
+-----
+Most tests should be added to the ash_unittests target. Tests that rely on
+//content should be added to ash_content_unittests, but these should be rare.
+
+Tests can bring up most of the ash UI and simulate a login session by deriving
+from AshTestBase. This is often needed to test code that depends on ash::Shell
+and the controllers it owns.
+
 Mus+ash
 ----------
-Ash is transitioning from using aura to using mus. During the
-transition period ash has support for both aura and mus. In order to
-work with both toolkits ash has a porting layer. This layer exists in
-ash/common. As portions of ash are converted to the porting layer they
-should move to ash/common. DEPS files may also be used to ensure new
-dependencies do not get added.
+Ash is transitioning to use the mus window server and gpu process, found in
+//services/ui. Ash continues to use aura, but aura is backed by mus. Code to
+support mus is found in //ash/mus. There should be relatively few differences
+between the pure aura and the aura-mus versions of ash. Ash can by run in mus
+mode by passing the --mus command line flag.
+
+Ash is also transitioning to run as a mojo service in its own process. This
+means that code in chrome cannot call into ash directly, but must use the mojo
+interfaces in //ash/public/interfaces.
+
+Out-of-process Ash is referred to as "mash" (mojo ash). In-process ash is
+referred to as "classic ash". Ash can run in either mode depending on the
+--mash command line flag.
+
+Historical notes
+----------------
+Ash shipped on Windows for a couple years to support Windows 8 Metro mode.
+Windows support was removed in 2016.