document "MIME leaf part" and "MIME tree" and "MIME part" and "MIME subtree"

[dkg]

terms about MIME structure are thrown around in the document without explicit definitions. maybe add to the Terminology section?

For example, "MIME leaf part" and "MIME tree" and "MIME part" and "MIME subtree"

[dkg]

Maybe also "MIME header" (as in, "the part of the MIME part where the headers go") wants an explicit definition?