gh-123138: Updated email.headerregistry docs to include required keyword parse_tree (GH-134450)

(cherry picked from commit 2c20f9c)

Co-authored-by: Gustaf <79180496+gostak-dd@users.noreply.github.com>

Author: gostak-dd

Doc/library/email.headerregistry.rst

@@ -96,9 +96,10 @@ headers.
    ``kwds`` is a dictionary containing one pre-initialized key, ``defects``.
    ``defects`` is an empty list.  The parse method should append any detected
    defects to this list.  On return, the ``kwds`` dictionary *must* contain
-   values for at least the keys ``decoded`` and ``defects``.  ``decoded``
-   should be the string value for the header (that is, the header value fully
-   decoded to unicode).  The parse method should assume that *string* may
+   values for at least the keys ``decoded``, ``defects`` and ``parse_tree``.
+   ``decoded`` should be the string value for the header (that is, the header
+   value fully decoded to unicode). ``parse_tree`` is set to the parse tree obtained
+   from parsing the header. The parse method should assume that *string* may
    contain content-transfer-encoded parts, but should correctly handle all valid
    unicode characters as well so that it can parse un-encoded header values.