[FAB-12454] Document collection policy details

Document relation between private data collection
policy and endorsement policy.

Change-Id: Iefe560ea014d2c2838f7c1de365e2ac169fc3dbe
Signed-off-by: David Enyeart <enyeart@us.ibm.com>

Author: denyeart

docs/source/private-data-arch.rst

@@ -22,9 +22,16 @@ Collection definitions are composed of five properties:
 
 * ``name``: Name of the collection.
 
-* ``policy``: Defines the organization peers allowed to persist the collection
-  data expressed using the ``Signature`` policy syntax, with each member being
-  included in an ``OR`` signature policy list.
+* ``policy``: The private data collection distribution policy defines which
+  organizations' peers are allowed to persist the collection data expressed using
+  the ``Signature`` policy syntax, with each member being included in an ``OR``
+  signature policy list. To support read/write transactions, the private data
+  distribution policy must define a broader set of organizations than the chaincode
+  endorsement policy, as peers must have the private data in order to endorse
+  proposed transactions. For example, in a channel with ten organizations,
+  five of the organizations might be included in a private data collection
+  distribution policy, but the endorsement policy might call for any three
+  of the organizations to endorse.
 
 * ``requiredPeerCount``: Minimum number of peers that the endorsing peer must
   successfully disseminate private data to before the peer signs the