FoG: Limit the comment field to 256 characters

That seems to be PayPal's limit: https://cms.paypal.com/uk/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_Appx_websitestandard_htmlvariables
FoG: Encourage the donor to write an email if 256 characters is not enough

We came up with that during GUADEC. If you have a better text, please suggest it.