Adds reminder in docs about resource ID as string

docs/customizing.rst

@@ -429,7 +429,8 @@ request::
     manager.create_api(Person, preprocessors=preprocessors)
 
 For preprocessors for endpoints of the form ``/api/person/1``, a returned value
-will be interpreted as the resource ID for the request. For example, if a
+will be interpreted as the resource ID for the request. (Remember, as described
+in :ref:`idstring`, the returned ID must be a string.) For example, if a
 preprocessor for a :http:method:`get` request to ``/api/person/1`` returns the
 string ``'foo'``, then Flask-Restless will behave as if the request were
 originally for the URL ``/api/person/foo``.  For preprocessors for endpoints of

docs/requestformat.rst

@@ -24,6 +24,19 @@ specification.
    updating
    updatingrelationships
 
+.. _idstring:
+
+Resource ID must be a string
+----------------------------
+
+As required by the JSON API, the ID (and type) of a resource must be a string
+in request and response documents. This does *not* mean that the primary key in
+the database must be a string, only that it will appear as a string in
+communications between the client and the server. For more information, see the
+`Identification`_ section of the JSON API specification.
+
+.. _Identification: http://jsonapi.org/format/#document-resource-object-identification
+
 .. _slashes:
 
 Trailing slashes in URLs