Elasticsearch moving java-api files requires temporary files left behind

To allow the ES docs build to continue to function [temporary file](https://github.com/elastic/elasticsearch/blob/7.x/server/src/test/java/org/elasticsearch/client/documentation/placeholder.txt) are needed to pacify the docs build. I think there is some logic in the build scripts merge that don't allow moving these directories.

I have a corresponding issue at https://github.com/elastic/elasticsearch/issues/60339