Explicitly state breaking modules as a concern with making settable f…

…ields read-only (#11426)

docs/content/develop/breaking-changes/breaking-changes.md

@@ -63,6 +63,8 @@ For more information, see
 * <a name="field-becoming-computed"></a> Making a settable field read-only
   * For MMv1 resources, adding `output: true` to an existing field.
   * For handwritten resources, adding `Computed: true` to a field that does not have `Optional: true` set.
+  * Even if there is no valid scenario where a field can be set, changing it to read-only may be a breaking change for
+    modules that depend on the provider.
 * <a name="field-oc-to-c"></a> Removing support for API-side defaults
   * For MMv1 resources, removing `default_from_api: true`.
   * For handwritten resources, altering a field schema with `Computed: true` + `Optional: true`