In a sharded search query that requires both the documents and meta cursor, we use the documents cursor on the shards, and the meta cursor on the merging node. Currently, explains do not do any work for the merging node. Because of this, we don't use the meta cursor for sharded search explains, and we also don't include the explain object from the meta cursor in our explain output. After SPM-3100 is done and we do obtain execution stats from the merging node, we should figure out how to include the explain object from the meta cursor in the explain output.