docs on causes/how to resolve jira.home being locked

Currently the jira.home directory can get locked if you configure 2 JIRA instances to use the same directory or one instance terminates abnormally and fails to unlock the home directory. In either of these cases we lock JIRA and prevent it from running. The explanation/resolution is a bit long to include in a log message so I'd prefer we have a web page somewhere and just provide a link to it.

After we get a URL for this we need to update JiraHomeStartupCheck.lockJiraHome to point to the correct URL.