DOCS-6880: Replica Set _id field must be numeric between 0-255

source/reference/method/rs.add.txt

@@ -58,14 +58,8 @@ Pass a Member Configuration Document to ``rs.add()``
 .. versionchanged:: 3.0.0
    Previous versions required an ``_id`` field in the document you
    passed to :method:`rs.add()`. After 3.0.0 you can omit the
-   ``_id`` field in this document.
-
-.. BEHAVIORAL NOTE:
-
-   If you specify the ``_id`` value, it must be unique. Use
-   :method:`rs.conf()` to see the existing ``_id`` values in the
-   replica set configuration document, and set ``_id`` to the next
-   unused ``_id`` value in the replica set.
+   ``_id`` field in this document. 
+   :rsconf:`members[n]._id` describes the requirements for specifying ``_id``. 
 
 The following operation adds a :program:`mongod` instance, running on
 the host ``mongodb4.example.net`` and accessible on the default port

source/reference/replica-configuration.txt

@@ -98,9 +98,10 @@ Replica Set Configuration Fields
 
       *Type*: integer
 
-      A numeric identifier of every member in the replica set. Once
-      set, you cannot change the
-      :rsconf:`members[n]._id` of a member.
+      An integer identifier of every member in the replica set. 
+      Values must be between 0 and 255 inclusive. Each replica set member 
+      must have a unique :rsconf:`_id<members[n]._id`>.  
+      Once set, you cannot change the :rsconf:`_id<members[n]._id>` of a member.
 
       .. note::