#14925: email now registers a defect for missing header/body separator.

This patch also deprecates the MalformedHeaderDefect.  My best guess is that
this defect was rendered obsolete by a refactoring of the parser, and the
corresponding defect for the new parser (which this patch introduces) was
overlooked.
diff --git a/Doc/library/email.errors.rst b/Doc/library/email.errors.rst
index d8f330f..b71ffa0 100644
--- a/Doc/library/email.errors.rst
+++ b/Doc/library/email.errors.rst
@@ -79,9 +79,18 @@
 * :class:`MisplacedEnvelopeHeaderDefect` - A "Unix From" header was found in the
   middle of a header block.
 
+* :class:`MissingHeaderBodySeparatorDefect` - A line was found while parsing
+  headers that had no leading white space but contained no ':'.  Parsing
+  continues assuming that the line represents the first line of the body.
+
+  .. versionadded: 3.3
+
 * :class:`MalformedHeaderDefect` -- A header was found that was missing a colon,
   or was otherwise malformed.
 
+  .. deprecated:: 3.3
+     This defect has not been used for several Python versions.
+
 * :class:`MultipartInvariantViolationDefect` -- A message claimed to be a
   :mimetype:`multipart`, but no subparts were found.  Note that when a message has
   this defect, its :meth:`is_multipart` method may return false even though its