fix(cloud_firestore): correct auto-generated ID documentation

[SONGBUILD]

Description

The API documentation for CollectionReference.add() and CollectionReference.doc() incorrectly stated that auto-generated IDs are prefixed with a client-generated timestamp and that the resulting list will be chronologically-sorted. This behavior is specific to Firebase Realtime Database (childByAutoId/push), not Cloud Firestore. Firestore auto-generated IDs are random strings with no timestamp component and no ordering guarantees.

Related Issues

Fixes #18422

Checklist

• I signed the CLA.
• The title of my PR starts with a [Conventional Commit] type (fix).
• My PR only addresses the documentation issue described in [📚] [cloud_firestore] Incorrect API documentation for document creation methods #18422.
• I read the [Contributor Guide] and followed the process outlined there.
• My changes only affect documentation comments (no code/behavior change).

Changes

• packages/cloud_firestore/cloud_firestore/lib/src/collection_reference.dart: Updated dartdoc for add() and doc() (2 occurrences).
• packages/cloud_firestore/cloud_firestore_platform_interface/lib/src/platform_interface/platform_interface_collection_reference.dart: Updated dartdoc for doc() (1 occurrence).

The same wording in firebase_database (Realtime Database) was left unchanged — for RTD, the timestamp-prefix description is correct.

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