Fix oximeter schema change comment (#9350)

Follow up to
#9341 (comment)

Author: karencfv

oximeter/db/schema/README.md

@@ -21,9 +21,11 @@ transactions, we actually require that there are no writes of any kind. In
 practice, this means `oximeter` **must not** be running when this is called.
 Similarly, there must be only a single instance of this program at a time.
 
-_NB: Schema changes for the `oximeter` database are currently disabled due to
-ongoing self-service update work. More information about this restriction can
-be found in [#8862](https://github.com/oxidecomputer/omicron/issues/8862)_
+_NB: Schema changes for the `oximeter` database are not allowed because there
+is no mechanism to apply them to existing systems. If we need to support schema
+changes at some point, we'll have to do the work to update them during automated
+update. See omicron [#8862](https://github.com/oxidecomputer/omicron/issues/8862)
+for details._
 
 To run this program:
 

oximeter/db/tests/integration_test.rs

@@ -43,9 +43,11 @@ impl TestInput {
 /// Ensure the oximeter database schemas for both single node and replicated
 /// cluster are not modified.
 ///
-/// Schema changes for the `oximeter` database are currently disabled due to
-/// ongoing self-service update work. More information about this restriction
-/// can be found in https://github.com/oxidecomputer/omicron/issues/8862
+/// Schema changes for the `oximeter` database are not allowed because there is
+/// no mechanism to apply them to existing systems. If we need to support schema
+/// changes at some point, we'll have to do the work to update them during
+/// automated update. See https://github.com/oxidecomputer/omicron/issues/8862
+/// for details.
 #[tokio::test]
 async fn test_schemas_are_not_modified() -> anyhow::Result<()> {
     let cur_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));