Document the ..1 and the "at most" behavior.

design/mvp/Explainer.md

@@ -1739,8 +1739,7 @@ in linear memory and the size in elements of the buffer. (See
 | Canonical ABI signature                      | `[future:i32 ptr:i32] -> [i32]`                                                     |
 
 where `read-status` is defined as in [`stream.read`](#-streamread)
-and `write-status` is defined as in [`stream.write`]. The number of elements
-returned when the value is `complete` is always `1`.
+and `write-status` is defined as in [`stream.write`].
 
 The `future.{read,write}` built-ins
 take the matching [readable or writable end](Async.md#streams-and-futures)
@@ -1750,6 +1749,11 @@ value was eagerly read or written, a sentinel indicating that the
 operation did not complete yet (`blocked`), or a sentinel indicating
 that the future is closed (`closed`).
 
+The number of elements returned when the value is `complete` is at most `1`.
+
+The `..1` in the buffer types indicates that these buffers may hold at most
+one element.
+
 In the Canonical ABI, the buffer is passed as a pointer to a buffer
 in linear memory.
 (See [`canon_future_read`] in the Canonical ABI explainer for details.)