Document POST policy upload versioning behavior (#11316)

S3-Object-Versioning.md

@@ -112,6 +112,23 @@ aws --endpoint-url $S3_ENDPOINT s3api delete-object \
 - **GET Object**: Returns the current version
 - **DELETE Object**: Permanently deletes the object
 
+### POST Policy Uploads:
+
+Uploads using an S3 POST policy, including presigned POST uploads, follow
+the bucket's versioning state:
+
+* Enabled: Creates a new version with a unique version ID and returns it
+  in the `x-amz-version-id` response header.
+* Suspended: Replaces the null version while preserving existing non-null
+  versions. The `x-amz-version-id` response header is omitted.
+* Never enabled: Uses the unversioned write path and omits the
+  `x-amz-version-id` response header.
+
+POST-created versions can be listed and retrieved using the same APIs
+shown above.
+
+Added in [PR #11316](https://github.com/seaweedfs/seaweedfs/pull/11316).
+
 ## Storage Layout
 
 SeaweedFS stores versioned objects using the following structure: