Update documentation for lacros.

Change-Id: I9f4d492117256fa55cb96c4dbbfe064d5daf9f35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2308190
Auto-Submit: Erik Chen <erikchen@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790080}

chrome/browser/chromeos/crosapi/README.md

 This directory contains the ash-chrome implementation of the ChromeOS API
-(crosapi) used by lacros-chrome. There are currently two types of files in this
-directory:
+(//chromeos/crosapi). This is the system implementation of ChromeOS-specific
+functionality which lacros-chrome requires.
+
+There are currently two types of files in this directory:
   * Files for launching and connecting to lacros-chrome. These are named
     lacros_foo.
   * Files that implement the crosapi. These are named foo_crosapi.

chromeos/crosapi/README.md

+This directory defines the ChromeOS API (crosapi). This is the
+communication protocol between lacros (web-browser on ChromeOS) and ash
+(user-space system executable on ChromeOS) for all new IPCs. Some existing IPCs
+might use Wayland or D-Bus to avoid unnecessary rewrites.
+

chromeos/lacros/README.md

-This directory contains the low-level system interfaces that lacros-chrome needs
-to function properly. This includes both device interaction (e.g. querying
-displays) and system-provided UI components (e.g. file picker).
+This directory contains the lacros-chrome client implementation of the ChromeOS
+API (//chromeos/crosapi). This is primarily glue code that allows lacros-chrome
+to access interfaces exposed by ash-chrome.
+
+Examples of contained functionality include:
+* Device interaction (e.g. querying displays)
+* System provided UI components (e.g. file picker)
 
 The closest equivalent on other operating systems would be a client-side library
 that provides OS-specific functionality, such as Win32 or Cocoa.