gh-61707: Clarify that Header.__str__() does not decode RFC 2047 encoded words - #161268

The docs for Header.__str__() say that all pieces are decoded, which suggests
that str(Header(s)) decodes RFC 2047 encoded words in s. It doesn't:

>>> from email.header import Header, decode_header, make_header
>>> str(Header('=?utf-8?q?Caf=C3=A9?='))
'=?utf-8?q?Caf=C3=A9?='
>>> str(make_header(decode_header('=?utf-8?q?Caf=C3=A9?=')))
'Café'

This adds a paragraph to the __str__() docs saying so, with an example
that points to decode_header() and make_header() for decoding raw
header strings.

An earlier PR for this issue (#31765) was closed without being merged. It
pointed readers to email.policy.default; this PR adds that pointer here too.

• Issue: [doc] email.header.Header.__unicode__ does not decode header #61707