fix header insertion to respect CRLF line-endings

844ca6b0 badly broke addmessageheader() because it changed the content
to use CRLF line-endings, so searching for the \n\n transition between
header and body always failed.  This resulted in mail files with the
following structure:

    X-OfflineIMAP: 3453842182-3083369391\n
    \n
    Subject: ...\n
    From: ...\n
    To: ...\n
    ... other header lines ...\n
    \n
    body line 1\n
    body line 2\n

Then MUAs and the IMAP server would treat this as a mail with a
single header line and a body which starts with something looking
suspiciously like a header.

Improved the docstring to clarify these issues.