Improve the documentation about how to use OperationsClient after FirestoreAdminClient.exportDocuments()

[icoloma]

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 generically documents name as "The name of the operation collection". The documentation in the proto file documents the expected format, but does not link to the format to be used when invoking OperationClient.listOperations.
• The javadoc for FirestoreAdminClient 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.