Clarify the API documentation for WebSQL database.

Bug: 8231270

WebSQL database is disabled by default. And setting change to
enable it must precede the first url navigation. This change
is to clarify this behavior.

Change-Id: Ic0aab764821daece83f3482b30df4bebc7ff1f37
diff --git a/core/java/android/webkit/WebSettings.java b/core/java/android/webkit/WebSettings.java
index 728bcd3..92b028e 100644
--- a/core/java/android/webkit/WebSettings.java
+++ b/core/java/android/webkit/WebSettings.java
@@ -1103,6 +1103,11 @@
      * false. See also {@link #setDatabasePath} for how to correctly set up the
      * database storage API.
      *
+     * This setting is global in effect, across all WebView instances in a process.
+     * Note you should only modify this setting prior to making <b>any</b> WebView
+     * page load within a given process, as the WebView implementation may ignore
+     * changes to this setting after that point.
+     *
      * @param flag true if the WebView should use the database storage API
      */
     public synchronized void setDatabaseEnabled(boolean flag) {