Clarify that the search box can now also find individual messages

from #2228 by @ianmacd

_locales/en/messages.json

@@ -320,7 +320,7 @@
   "beginYourSession": "Begin your Session.",
   "welcomeToYourSession": "Welcome to your Session",
   "newSession": "New Session",
-  "searchFor...": "Search for conversations or contacts",
+  "searchFor...": "Search for messages or conversations",
   "enterSessionID": "Enter Session ID",
   "enterSessionIDOfRecipient": "Enter Session ID or ONS name of recipient",
   "usersCanShareTheir...": "Users can share their Session ID by going into their account settings and tapping \"Share Session ID\", or by sharing their QR code.",