Document that serde_json allows sequences to be deserialized into structs

[tbu-]

NOTE: This is basically #2438 which was closed due to being a duplicate of #1587. #1587 is about changing that behavior (which is probably impossible given serde's backward compatibility guarantees). This issue (and the closed #2438) are about documenting the existing behavior.

Last time I saw this was https://old.reddit.com/r/rust/comments/1332t9b/ji7oo8m/, but I've also seen it earlier and found it surprising both times.

Basically both {"hello": "test", "value": 42} (expected) and ["test", 42] deserialize perfectly fine into the following struct:

#[derive(Serialize, Deserialize)]
pub struct HelloParams {
    hello: String, 
    value: i32,
}

Since this is not mentioned anywhere, this can accidentally become a used API when serde is used to parse JSON (Hyrum's law).

It would be nice if this could be featured somewhere prominently in the docs. I couldn't actually find it there, even looking for it.