Somewhere along the way, the body-txt and body-html fields were merged
into a single body field. Later, commit 8eac392221 ("guile: re-enable
the guile support") finally removed support for Field::Id::BodyHtml from
mu:c:get-field.
Unfortunately mu.scm and the documentation are still stuck in the past, so
update them. mu:body-txt is now a synonym for mu:body, and mu:body-html
always returns #f.
I wanted to add a mu:body test also for the rfc822.1 message, but there's
currently a bug where its body text is is duplicated (issue #2802), so the
test would fail.
