doc: Update schema README to drop rules.dataset_metadata

Author: effigies

src/schema/README.md

@@ -873,9 +873,9 @@ meg_meg_common:
     description: optional
 ```
 
-### Sidecar and tabular data rules
+### JSON and tabular data rules
 
-Tabular data and JSON sidecar files follow a similar pattern:
+Tabular data and JSON files follow a similar pattern:
 
 |      | Name          | Value         |
 | ---- | ------------- | ------------- |
@@ -937,7 +937,7 @@ rule is applied.
 
 #### Valid fields for definitions
 
-1.  `rules.sidecars.*`
+1.  `rules.sidecars.*` and `rules.json.*`
 
     | Field       | Description                                                                                              |
     | ----------- | -------------------------------------------------------------------------------------------------------- |
@@ -984,6 +984,11 @@ MRIFuncVolumeTiming:
 An additional check will be required to assert that both are not present,
 but these tables may be combined for rendering purposes.
 
+`rules.json.*` and `rules.sidecars.*` are essentially identical,
+except `rules.json` rules describe the contents of individual JSON files,
+while `rules.sidecars` rules describe the contents of sidecar files,
+accumulated by the inheritance principle.
+
 Here we present an example rule in `rules.tabular_data.eeg`:
 
 ```YAML
@@ -1058,9 +1063,6 @@ EventsMissing:
     | ----------- | ------------------------------------- |
     | `datatypes` | List of datatypes mapping to modality |
 
--   `rules.dataset_metadata` - These are similar to `rules.sidecars.*`, for JSON files at the root level.
-    This is likely to go away in favor of other approaches.
-
 -   `rules.errors` - This file describes errors that cannot be expressed in the schema. This provides common
     codes and language that implementing validators can use to ensure the same problems are reported to
     users in the same way.