PWA: add some doc about Policy and System apps

This adds a one liner for policy and system web apps to explain what
they are and used for.

Bug: none
Change-Id: I3d656b91789e013b7535aa58ed541dda43023675
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353524
Commit-Queue: Phillis Tang <phillis@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797914}

chrome/browser/web_applications/components/policy/web_app_policy_manager.h

@@ -22,9 +22,11 @@ class PrefRegistrySyncable;
 
 namespace web_app {
 
-// Tracks the policy that affects Web Apps and also tracks which Web Apps are
-// currently installed based on this policy. Based on these, it decides which
-// apps to install, uninstall, and update, via a PendingAppManager.
+// Policy installation allows enterprise admins to control and manage
+// Web Apps on behalf of their managed users. This class tracks the policy that
+// affects Web Apps and also tracks which Web Apps are currently installed based
+// on this policy. Based on these, it decides which apps to install, uninstall,
+// and update, via a PendingAppManager.
 class WebAppPolicyManager {
  public:
   static constexpr char kInstallResultHistogramName[] =

chrome/browser/web_applications/system_web_app_manager.h

@@ -122,6 +122,8 @@ struct SystemAppInfo {
 };
 
 // Installs, uninstalls, and updates System Web Apps.
+// System Web Apps are built-in, highly-privileged Web Apps for Chrome OS. They
+// have access to more APIs and are part of the Chrome OS image.
 class SystemWebAppManager {
  public:
   // Policy for when the SystemWebAppManager will update apps/install new apps.