guile: Fix the mu:body message method

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.

guile/examples/org2mu4e

@@ -43,7 +43,7 @@ exec guile -e main -s $0 $@
     (org-mu4e-link msg)
     (if tag (string-concatenate `(":" ,tag "::")) "")
     (or (mu:from msg) "?")
-    (let ((body (mu:body-txt msg)))
+    (let ((body (mu:body msg)))
       (if (not body) ;; get a 'summary' of the body text
 	"<no plain-text body>"
 	(string-map