Document future-read-result.cancelled's Canonical ABI encoding. (#542)

* Document `future-read-result.cancelled`'s Canonical ABI encoding.

In the Canonical ABI, `future-read-result` has the same encoding as
`future-write-result`, even though the `enum`s defined in the
documentation have `cancelled` at different indices. Add a sentence
to the documentation mentioning this.

* Update design/mvp/Explainer.md

Co-authored-by: Luke Wagner <mail@lukewagner.name>

---------

Co-authored-by: Luke Wagner <mail@lukewagner.name>

Author: sunfishcode

design/mvp/Explainer.md

@@ -1919,7 +1919,9 @@ In the Canonical ABI, the `{readable,writable}-future-end` is passed as an
 `i32` describing the linear memory offset of the
 `{readable,writable}-buffer<T; 1>`. The `option<future-{read,write}-result>`
 return value is bit-packed into the single `i32` return value where
-`0xffff_ffff` represents `none`.
+`0xffff_ffff` represents `none`. And, `future-read-result.cancelled` is encoded
+as the value of `future-write-result.cancelled`, rather than the value implied
+by the `enum` definition above.
 
 (See [`canon_future_read`] in the Canonical ABI explainer for details.)