[FAB-8738] Document Reserved Field Names in CouchDB

This patch add description of reserved field names in a JSON
document to be put a CouchDB state database.

Change-Id: I679a9de8cd73f1c4dc8b3a104d8e2a5e6657362e
Signed-off-by: Yuta Namiki <y-namiki@ak.jp.nec.com>

Author: Yuta Namiki

docs/source/couchdb_as_state_database.rst

@@ -37,6 +37,14 @@ default embedded LevelDB, and move to CouchDB if you require the additional comp
 It is a good practice to model chaincode asset data as JSON, so that you have the option to perform
 complex rich queries if needed in the future.
 
+.. note:: A JSON document cannot use the following field names at the top level.
+   These are reserved for internal use.
+
+   - ``_deleted``
+   - ``_id``
+   - ``_rev``
+   - ``~version``
+
 Using CouchDB from Chaincode
 ----------------------------