#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.generator.rst b/Doc/library/email.generator.rst
index 22d8b09..85b32fe 100644
--- a/Doc/library/email.generator.rst
+++ b/Doc/library/email.generator.rst
@@ -79,8 +79,8 @@
 
       Messages parsed with a Bytes parser that have a
       :mailheader:`Content-Transfer-Encoding` of 8bit will be converted to a
-      use a 7bit Content-Transfer-Encoding.  Any other non-ASCII bytes in the
-      message structure will be converted to '?' characters.
+      use a 7bit Content-Transfer-Encoding.  Non-ASCII bytes in the headers
+      will be :rfc:`2047` encoded with a charset of `unknown-8bit`.
 
       .. versionchanged:: 3.2
          Added support for re-encoding 8bit message bodies, and the *linesep*