Update reindex.asciidoc
Highlighted that `_source` needs to exist in the source index.
[[docs-reindex]]
== Reindex API
+IMPORTANT: Reindex requires <<mapping-source-field,`_source`>> to be enabled for
+all documents in the source index.
+
IMPORTANT: Reindex does not attempt to set up the destination index. It does
not copy the settings of the source index. You should set up the destination
index prior to running a `_reindex` action, including setting up mappings, shard
