Update splitting JIRA instances doc to mention attachments

Need to update:
http://www.atlassian.com/software/jira/docs/latest/splitting.html

The document should mention that when splitting a JIRA instance there are 2 things to worry about:

1. data
2. attachments

The doc already covers data, but needs to talk about attachments.

JIRA keeps attachments on the file system. When splitting instances, the attachment directory will need to be duplicated. After restoring the data on the second JIRA instance, need to login as admin user and repoint the second instance of JIRA at the copy of the attachment directory.

When projects are deleted from an instance of JIRA, the relevant attachments (i.e. attachments that belong to issues for projects that are being deleted) will be removed.