Improve pop-up help when importing event participants

I spent several hours debugging code trying to work out why the External ID field wasn't available when importing event participants (see http://forum.civicrm.org/index.php/topic,24949.msg105209.html)

In the end, it was as simple as not being able to use the "update" option, but stupidly I never tried the other options until I'd spent a lot of time on the problem.

I have provided some minor updates to the pop-up help so that hopefully nobody else will run into this situation.

The attached file patches against civicrm/templates/CRM/Event/Import/Form/UploadFile.hlp

Thanks,

Andy