Add notes

docs/notes/2.22.x.md

@@ -22,6 +22,19 @@ The two systems are expected to be more-or-less functionally identical. We plan
 
 If you encounter such discrepancies, and you can't resolve them easily, please [reach out to us on Slack or file an issue](https://www.pantsbuild.org/community/getting-help).
 
+### Fine grained diff with line numbers
+
+This release introduces `Target.origin_sources_blocks` field that allows any
+plugin to define dependencies on individual blocks of code rather that the
+whole file. The `--changed-since` logic was updated  to calculate fine grained
+diff based on line numbers and compare the changed lines to
+`origin_sources_blocks` to figure out which targets changed.
+
+This feature is not used by any real plugin yet, but you can look at the [test
+plugin](https://github.com/pantsbuild/pants/tree/2.22.x/testprojects/pants-plugins/src/python/python_constant/target_types.py)
+for inspiration. To opt into the feature set the flag
+`--enable-target-origin-sources-blocks`.
+
 ### Backends
 
 #### NEW: SQL

testprojects/pants-plugins/src/python/python_constant/target_types_test.py

@@ -29,13 +29,11 @@ def rule_runner():
             *module_mapper.rules(),
             *stripped_source_files.rules(),
             *target_types_rules.rules(),
-            # python_register.rules(),
             QueryRule(Addresses, (DependenciesRequest,)),
         ],
         target_types=[
             PythonSourceTarget,
             PythonSourcesGeneratorTarget,
-            # *python_register.target_types(),
             PythonConstantTarget,
             PythonConstantTargetGenerator,
         ],