add warnings to reset client/server data buttons in the prefs

services/sync/locales/en-US/preferences.properties

@@ -1,2 +1,6 @@
 # %S is the username of the signed in user
 signedIn.description = Signed in as %S
+reset.server.warning.title = Warning
+reset.server.warning = This will delete all data on the server.\n\nYou *must* restart this and any other instances of Firefox you may have running (on any computer).
+reset.client.warning.title = Warning
+reset.client.warning = This will delete all bookmarks and history data.  Are you sure you want to do this?