Note correct usage of $edit_headers. (closes #3673)

Note correct usage of $edit_headers. (closes #3673)

Ticket 3673 shows that it isn't necessarily obvious a blank
line is needed between the headers and body of a message when using
$edit_headers.  This patch adds a small note to the documentation,
indicating the blank line should be preserved between the headers and
body.

Thank you Sinan Kaan Yerli for the patch.