Revise prose for CiviCase notifications

CiviHR will periodically send email reminders about important steps in the exiting, joining, and probation processes. The current prose for those emails is quite spartan and should be revised.

To see a list of email reminders and their prose, one can login to the demo site and navigate to "Administer => Communication => Schedule Reminders".

Please prepare new prose, attach the new prose to this issue, and reassign to Yash.