api [nfc]: Describe getSingleMessage error cases more explicitly

In particular make it clear that while this function can return void,
that is not what it does in the case where the message doesn't exist.

(Based on going and reading the linked server API doc.)

Author: gnprice

src/api/messages/getSingleMessage.js

@@ -22,8 +22,11 @@ type ServerApiResponseSingleMessage = {|
 /**
  * See https://zulip.com/api/get-message
  *
- * Gives undefined if the `message` field is missing, which it will be for
- * FL <120.
+ * Throws an error if the message doesn't exist at all, or isn't visible to
+ * our user.
+ *
+ * Otherwise, gives undefined on old servers (FL <120) where this API
+ * endpoint doesn't return a `message` field.
  */
 // TODO(server-5.0): Simplify FL-120 condition in jsdoc and implementation.
 export default async (