#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.header.rst b/Doc/library/email.header.rst
index ff2b484..29752c4 100644
--- a/Doc/library/email.header.rst
+++ b/Doc/library/email.header.rst
@@ -130,8 +130,14 @@
 
    .. method:: __str__()
 
-      A helper for :class:`str`'s :func:`encode` method.  Returns the header as
-      a Unicode string.
+      Returns an approximation of the :class:`Header` as a string, using an
+      unlimited line length.  All pieces are converted to unicode using the
+      specified encoding and joined together appropriately.  Any pieces with a
+      charset of `unknown-8bit` are decoded as `ASCII` using the `replace`
+      error handler.
+
+      .. versionchanged:: 3.2
+         Added handling for the `unknown-8bit` charset.
 
 
    .. method:: __eq__(other)