Update Repeatability headers section

Author: JeffreyRichter

azure/Guidelines.md

@@ -17,6 +17,7 @@ Please ensure that you add an anchor tag to any new guidelines that you add and
 | Date        | Notes                                                          |
 | ----------- | -------------------------------------------------------------- |
 | 2023-May-12 | Explain service response for missing/unsupported `api-version` |
+| 2023-Apr-21 | Update/clarify guidelines on POST method repeatability         |
 | 2023-Apr-07 | Update/clarify guidelines on polymorphism                      |
 | 2022-Sep-07 | Updated URL guidelines for DNS Done Right                      |
 | 2022-Jul-15 | Update guidance on long-running operations                     |
@@ -836,13 +837,13 @@ For example:
 <a href="#repeatability" name="repeatability"></a>
 ### Repeatability of requests
 
-The ability to retry failed requests for which a client never received a response greatly simplifies the ability to write resilient distributed applications. While HTTP designates some methods as safe and/or idempotent (and thus retryable), being able to retry other operations such as create-using-POST-to-collection is desirable.
-
-<a href="#repeatability-headers" name="repeatability-headers">:ballot_box_with_check:</a> **YOU SHOULD** support repeatable requests according as defined in [OASIS Repeatable Requests Version 1.0](https://docs.oasis-open.org/odata/repeatable-requests/v1.0/repeatable-requests-v1.0.html).
+Fault tolerant applications require that clients retry requests for which they never got a response, and services must handle these retried requests idempotently. In Azure, all HTTP operations are naturally idempotent except for POST used to create a resource and [POST when used to invoke an action](
+https://github.com/microsoft/api-guidelines/blob/d81151d990b437b34dcf7d05d9504b754eb4f456/azure/Guidelines.md#performing-an-action). 
 
+<a href="#repeatability-headers" name="repeatability-headers">:ballot_box_with_check:</a> **YOU SHOULD** support repeatable requests as defined in [OASIS Repeatable Requests Version 1.0](https://docs.oasis-open.org/odata/repeatable-requests/v1.0/repeatable-requests-v1.0.html) for POST operations to make them retriable.
 - The tracked time window (difference between the `Repeatability-First-Sent` value and the current time) **MUST** be at least 5 minutes.
-- A service advertises support for repeatability requests by adding the `Repeatability-First-Sent` and `Repeatability-Request-ID` to the set of headers for a given operation.
-- When understood, all endpoints co-located behind a DNS name **MUST** understand the header. This means that a service **MUST NOT** ignore the presence of a header for any endpoints behind the DNS name, but rather fail the request containing a `Repeatability-Request-ID` header if that particular endpoint lacks support for repeatable requests. Such partial support **SHOULD** be avoided due to the confusion it causes for clients.
+- Document the POST operation's support for the `Repeatability-First-Sent`, `Repeatability-Request-ID`, and `Repeatability-Result` headers in the API contract and documentation.
+- Any operation that does not support repeatability headers should return a 501 (Not Implemented) response for any request that contains valid repeatability request headers.
 
 <a href="#lro" name="lro"></a>
 ### Long-Running Operations & Jobs