Change to guidelines based on Mike Pizzo feedback

Author: Dan Kershaw [MSFT]

graph/GuidelinesGraph.md

@@ -250,7 +250,7 @@ If possible, APIs SHOULD use resource-based designs with standard HTTP methods r
 
 |  Microsoft Graph rules for modeling behavior                     |
 |------------------------------------------------------------------|
-| :heavy_check_mark: **MUST** use POST to create new entities in insertable entity sets or collections.<BR>This approach requires the server to produce a system generated key. |
+| :heavy_check_mark: **MUST** use POST to create new entities in insertable entity sets or collections.<BR>This approach requires the service to produce a system-generated key, or for a caller to provide a key in the request payload. |
 | :ballot_box_with_check: **SHOULD** additionally use PATCH to create new entities in insertable entity sets or collections.<BR>This [Upsert](./patterns/upsert.md) approach requires the caller to provide a key. |
 | :heavy_check_mark: **MUST** use PATCH to edit updatable resources.  |
 | :heavy_check_mark: **MUST** use DELETE to delete deletable resources. |