#10686: recode non-ASCII headers to 'unknown-8bit' instead of ?s.

This applies only when generating strings from non-RFC compliant binary
input; it makes the existing recoding behavior more consistent (ie:
now no data is lost when recoding).
diff --git a/Doc/library/email.message.rst b/Doc/library/email.message.rst
index e76e689..29f7ba3 100644
--- a/Doc/library/email.message.rst
+++ b/Doc/library/email.message.rst
@@ -169,9 +169,10 @@
    Note that in all cases, any envelope header present in the message is not
    included in the mapping interface.
 
-   In a model generated from bytes, any header values that (in contravention
-   of the RFCs) contain non-ASCII bytes will have those bytes transformed
-   into '?' characters when the values are retrieved through this interface.
+   In a model generated from bytes, any header values that (in contravention of
+   the RFCs) contain non-ASCII bytes will, when retrieved through this
+   interface, be represented as :class:`~email.header.Header` objects with
+   a charset of `unknown-8bit`.
 
 
    .. method:: __len__()