Document why `XdsTestClient.check_channel_successful_calls` is not using `calls_succeeded`

Context: https://github.com/grpc/grpc/pull/35280#issuecomment-1851100790

~Not sure why we didn't notice [`calls_succeeded`](https://github.com/grpc/grpc/blob/b63a21b419040506df091854c45654dadbf822f2/src/proto/grpc/channelz/channelz.proto#L111-L112) in the first place.~
~I tried the oldest builds of xds test clients of each language, and all of them correctly set this field.~

See https://github.com/grpc/psm-interop/issues/13#issuecomment-1852532561.

cc @yashykt