WRITE_APN_SETTINGS is also required to read

Add documentation explaining this.

Bug: 110033164
Test: no change to behavior
Change-Id: I4b69e5e36c9e05cf14c999e4a5bb1664a02e82f3
diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 64949ce..7877e79 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -2287,7 +2287,8 @@
     <permission android:name="android.permission.ASEC_RENAME"
         android:protectionLevel="signature" />
 
-    <!-- @SystemApi Allows applications to write the apn settings.
+    <!-- @SystemApi Allows applications to write the apn settings and read sensitive fields of
+         an existing apn settings like user and password.
     <p>Not for use by third-party applications. -->
     <permission android:name="android.permission.WRITE_APN_SETTINGS"
         android:protectionLevel="signature|privileged" />