Improve help messages on modals

Some general guidelines:
1. Removed contractions
2. Changed "Info" to "Information"
3. Removed the "From here you can..." at the beginning of messages.
4. Explained concepts to users instead of just reiterating the modal
title. Thanks Jeff Calcaterra!

Co-Authored-By: Cindy Lu <clu@us.ibm.com>
Closes-Bug: #1329984

Change-Id: I35af81953cafcbafc28c31b3ce305e8c5e21fe84
Merge "Improve help messages on modals"
Merge branch 'master' of https://github.com/openstack/horizon