Clarify index operations for create privilege (elastic/x-pack-elasticsearch#4337)

The `create` privilege can be misleading based on a user's
interpretation of the meaning of the privilege. The create privilege
grants the user access to the index api and the put mapping api. The
index api allows for a document to be created but it also allows for
an existing document to be overwritten (assuming the CREATE operation
type is not used). However, the name `create` implies the ability to
only create a document and not be able to modify an existing document,
which may not be clear to users unfamiliar with the index API.

This commit adds a small note under the privilege in the documentation
that should add some clarity to the operations allowed by this
privilege.

Original commit: elastic/x-pack-elasticsearch@24596c41b0

docs/en/security/reference/privileges.asciidoc

@@ -100,6 +100,13 @@ mapping action.
 `create`::
 Privilege to index documents. Also grants access to the update mapping
 action.
++
+--
+NOTE: This privilege does not restrict the index operation to the creation
+of documents but instead restricts API use to the index API. The index API allows a user
+to overwrite a previously indexed document.
+
+--
 
 `delete`::
 Privilege to delete documents.