[docs] FirestoreAdminClient.exportDocuments() no longer in docs

v6.5.0 of the docs has [a section for exportDocuments()](https://googleapis.dev/nodejs/firestore/6.5.0/google.firestore.admin.v1.FirestoreAdmin.html#exportDocuments1)

v6.6.0 and beyond does not.

But it seems that the APIs are still in the types and the proto code? For example, [here](https://github.com/googleapis/nodejs-firestore/blob/7a9d17bdd5fca3d91e66f7500c035140a7397c33/dev/protos/firestore_admin_v1_proto_api.js#L1046-L1057), [here 2](https://github.com/googleapis/nodejs-firestore/blob/7a9d17bdd5fca3d91e66f7500c035140a7397c33/types/v1/firestore_admin_client.d.ts#L551-L626) and [here 3](https://github.com/googleapis/nodejs-firestore/blob/7a9d17bdd5fca3d91e66f7500c035140a7397c33/types/protos/firestore_admin_v1_proto_api.d.ts#L324-L343)

So the docs should be updated to reflect that the API still exists, or the [CHANGELOG](https://github.com/googleapis/nodejs-firestore/blob/main/CHANGELOG.md) should indicate that this *breaking change* exists.

Compare the v6.5.0 exportDocuments() documentation with the generated API references linked in dev/protos/firestore_admin_v1_proto_api.js, types/v1/firestore_admin_client.d.ts, and types/protos/firestore_admin_v1_proto_api.d.ts. Determine whether the API is still intended to be documented, then update the documentation or the CHANGELOG to clearly record the outcome.

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