document the ability to customise the Subject of JIRA emails

See Peter L's FedEx13 project: https://extranet.atlassian.com/display/~pleschev/2010/01/18/Fedex+13+Delivery+-+JRA-3609+-+Making+224+users+happy

=> TO DO: update this page in the Admin Guide: http://confluence.atlassian.com/display/JIRA/Customising+Email+Content
=> REPLACE WITH CONTENTS OF THIS PAGE: http://confluence.atlassian.com/display/JIRA/Customising+Email+Content+%28copy+for+4.1%29

=> DONE: update this page in the Developer docs: http://confluence.atlassian.com/display/JIRA/Velocity+Context+for+Email+Templates
http://confluence.atlassian.com/pages/diffpages.action?pageId=166978&originalId=214864052