document new location of log files

As of 4.1, JIRA no longer logs in your working directory. All its logs get output into the $JIRA_HOME/log directory.

(see JRA-19173)

TO DO:
Update these pages:
=> http://confluence.atlassian.com/display/JIRA/JIRA+Home+Directory
=> http://confluence.atlassian.com/pages/viewpage.action?pageId=16121981 ('Where are the JIRA logs')
=> http://confluence.atlassian.com/display/JIRA/Running+JIRA+Standalone+as+a+Service

TO DO:
=> Update this FAQ:
http://confluence.atlassian.com/x/fQD2