Settings: update documentation for REQUIRE_PASSWORD_TO_DECRYPT

This setting no longer does anything, so update its documentation.  I'd
rather just remove it entirely, or at least deprecate it, but it's
unclear that either is allowed by the API guidelines.

Bug: 208476087
Change-Id: I031e9f4316d76634350deea211767e8390e5c4f3
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 3886708..f4b8aa9 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -15711,8 +15711,13 @@
         public static final String NETWORK_SCORING_PROVISIONED = "network_scoring_provisioned";
 
         /**
-         * Indicates whether the user wants to be prompted for password to decrypt the device on
-         * boot. This only matters if the storage is encrypted.
+         * On devices that use full-disk encryption, indicates whether the primary user's lockscreen
+         * credential is required to decrypt the device on boot.
+         * <p>
+         * This setting does not do anything on devices that use file-based encryption.  With
+         * file-based encryption, the device boots without a credential being needed, but the
+         * lockscreen credential is required to unlock credential-encrypted storage.  All devices
+         * that launched with Android 10 or higher use file-based encryption.
          * <p>
          * Type: int (0 for false, 1 for true)
          *