CRM-12325 Corrected erroneous help info on message template page and form, and consolidated the help. Also added help and link to book on Scheduled Reminders page.

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* CRM-12325: Correct and clarify help regarding use of tokens for events, memberships, and activities in message templates
  http://issues.civicrm.org/jira/browse/CRM-12325
Merge pull request #438 from davecivicrm/CRM-12325

CRM-12325 Corrected erroneous help info on message template page and for...