Update reindex.asciidoc (#35062)

Highlighted that `_source` needs to exist in the source index.
2018-10-30 20:22:42 +11:00 · 2018-10-30 20:22:42 +11:00 · 5e0c9b7c1a
parent e0fef52058
commit 5e0c9b7c1a
1 changed files with 3 additions and 0 deletions
--- a/docs/reference/docs/reindex.asciidoc
+++ b/docs/reference/docs/reindex.asciidoc
@ -1,6 +1,9 @@
 [[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