This update looks great to me. A few comments:
"with lines of no more than 997 characters (by convention usually no
more than 80)"
should be "with lines of no more than 998 characters (by convention
usually no more than 78)" to match Section 2.1.1 of RFC 2822.
"Numerous software products erroneously label this media type as
Text/Plain, resulting in much user discomfort."
Media type has a specific meaning in MIME that I don't think you want to
imply here. I would suggest replacing "media type" with "kind of
textual content".
In section 5.1, I would replace both 80 and 79 with 78, as per RFC 2822.
- dan
--
Dan Kohn <mailto:dan(_at_)dankohn(_dot_)com>
<http://www.dankohn.com/> <tel:+1-650-327-2600>