[FR]: Add `@discardableResult` to `CollectionReference.addDocument`

[Eskils]

Description

Currently, when adding an Encodable document to a collection, the addDocument method returns a DocumentReference. Often this is not need and thus generates a warning in Xcode which can be silenced by

let _ = try Firestore.firestore().collection("collectionName").addDocument(from: encodable)

To improve developer experience I suggest to add the @discardableResult attribute to the addDocument method which will tell the Swift compiler that it is okay for the return value to be unused.

See required changes here: https://github.com/Eskils/firebase-ios-sdk/commit/c44c187f92787b9978ba73e4bbbda14d6a78c665

API Proposal

No response

Firebase Product(s)

Firestore

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.

[wu-hui]

If you want, you can create a PR. Otherwise, I can make the change.

Thanks!