Add description for android:canRetrieveWindowContent

An AccessibilityService would need both capability and flag for
retrieving content of all interactive windows.

Bug: 122497116
Test: build pass
Change-Id: I3483a250a8fb8fef69f33b4e8e170a477e8a5ca0
diff --git a/core/res/res/values/attrs.xml b/core/res/res/values/attrs.xml
index 91faa55..2ba3177 100644
--- a/core/res/res/values/attrs.xml
+++ b/core/res/res/values/attrs.xml
@@ -3660,7 +3660,12 @@
              the settings for this service. This setting cannot be changed at runtime. -->
         <attr name="settingsActivity" />
         <!-- Attribute whether the accessibility service wants to be able to retrieve the
-             active window content. This setting cannot be changed at runtime. -->
+             active window content. This setting cannot be changed at runtime.
+             <p>
+             Required to allow setting the {@link android.accessibilityservice
+             #AccessibilityServiceInfo#FLAG_RETRIEVE_INTERACTIVE_WINDOWS} flag.
+             </p>
+         -->
         <attr name="canRetrieveWindowContent" format="boolean" />
         <!-- Attribute whether the accessibility service wants to be able to request touch
              exploration mode in which touched items are spoken aloud and the UI can be