Update reindex.asciidoc (#35062)
Update reindex.asciidoc (#35062)

Highlighted that `_source` needs to exist in the source index.
vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra