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.

**NOTE: I am not sure whether other sub-classes of `BaseFolder` also need `linebreak=CRLF` adding to the `addmessageheader()` call?**