[FAB-10571] Documentation for CouchDB field names

Ledger removed the "data" wrapper when using CouchDB as the
state database.  This change means that documents in the state
database must meet CouchDB's restrictions for top level field
names.

The documentation is being updated to clarify that the following
field names cannot be used at the top level:

- Any field beginning with an underscore, "_"
- ~version

Change-Id: Ie2db9590e6eae81bca07974c720ea97493486366
Signed-off-by: Chris Elder <chris.elder@us.ibm.com>

docs/source/couchdb_as_state_database.rst

@@ -40,9 +40,7 @@ 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``
+   - ``Any field beginning with an underscore, "_"``
    - ``~version``
 
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