Add warning in Mail Archive that adding mail account will delete email even for IMAP

Related to CONF-7104 users can accidentally delete all their email from their IMAP account when adding the mail account to Confluence via

http://confluence.atlassian.com/display/DOC/Adding+a+Mail+Account

Most users will assume that IMAP would mean their email would not be deleted and would not read the above documentation which has the warning:

Configured accounts (including IMAP) will have their mail downloaded and removed from the server. For more information, please refer to our online documentation.

We should add this warning to the Add Mail Archive page within Confluence.