[java-firestore] Improve the documentation about how to use OperationsClient after FirestoreAdminClient.exportDocuments()

Apologies if I misread the documentation, but I think I found several gaps in the documentation trying to query operations after launching `FirestoreAdminClient.exportDocuments`. Most of the issues are caused because the way to use `OperationsClient` specifically to track Firestore Operations is not really documented (or I couldn't find it).

* The [javadoc for OperationsClient](https://github.com/googleapis/gax-java/blob/878bcf2da215e14d9e3bcb4717b85d867f149cee/gax-grpc/src/main/java/com/google/longrunning/OperationsClient.java#L243) generically documents `name` as "The name of the operation collection". The documentation [in the proto file](https://github.com/googleapis/java-firestore/blob/aabd4ac4e43e1c58ec25e51117980f4396a3e8d9/proto-google-cloud-firestore-admin-v1/src/main/proto/google/firestore/admin/v1/firestore_admin.proto#L417) documents the expected format, but does not link to the format to be used when invoking `OperationClient.listOperations`.
* The [javadoc for FirestoreAdminClient](https://github.com/googleapis/java-firestore/blob/3aea7bcfaebb493c1397a6856be92783700dbca4/google-cloud-firestore-admin/src/main/java/com/google/cloud/firestore/v1/FirestoreAdminClient.java#L94) says that "by default, ListOperations will only return in progress and failed operations" but it does not describe what a running / successful / failed operation looks like, and particularly how to identify a failed operation.
* The way to unpack `ExportDocumentsMetadata` from an `Operation` is not documented. I'm using `op.getMetadata().unpack(ExportDocumentsMetadata.class)`.
* I could not find any documentation about supported filters when invoking `OperationsClient.listOperations`. It's only documented that one can use `done:true`. For example, all variants that I could try of searching for `metadata.@type:type.googleapis.com/google.firestore.admin.v1.ExportDocumentsMetadata` did not work, so I suppose that's not supported. It would be good to know what is supported.