Update rules/rest-api-design/rule.md

Co-authored-by: Daniel Mackay [SSW] <[email protected]>

rules/rest-api-design/rule.md

@@ -55,7 +55,9 @@ This provides a much more consistent API structure when querying both collection
 
 ### 3. Idempotent requests
 
-Design `GET`, `PUT` and `DELETE` operations as idempotent, where repeating an action yields the same result as performing it once. This avoids unintended actions from repeated requests.
+Design requests as idempotent operations, where repeating an action yields the same result as performing it once. This avoids unintended actions from repeated requests.
+
+Typically, `GET`, `PUT` and `DELETE` are idempotent by default, but special handling needs to be added to make `POST` requests idempotent.
 
 ::: greybox
 If a `DELETE` request removes a record, re-sending it should not throw errors if the record is already deleted.