Update serialization.md to include deprecation of serializable values (#4411)

I had to deprecate a serializable value in #4398 and couldn't find documentation around how to do it. This PR adds the relevant documentation.

Author: tanujkhattar

cirq-core/cirq/testing/json.py

@@ -53,7 +53,8 @@ class ModuleJsonTestSpec:
     should_not_be_serialized: List[str]
     # points to the resolver cache's dict for this module
     resolver_cache: Dict[str, ObjectFactory]
-    # deprecations
+    # {DeprecatedClass: deprecation_deadline} pairs to avoid deprecation errors
+    # in serialization tests.
     deprecated: Dict[str, str]
 
     def __repr__(self):

docs/dev/serialization.md

@@ -113,6 +113,18 @@ Ideally, the contents of the `.repr` file are exactly the output of
 `your_class_name.json` should contain the expected JSON output when serializing
 the test value from `your_class_name.repr`.
 
+## Deprecating a serializable value
+When a serializable value is marked deprecated, but is not yet removed, the 
+`.json` and `.repr` files continue to exist but `json_serialization_test.py` 
+will start complaining that deprecated values cannot be used in tests.     
+In order to fix this, one should add an entry corresponding to deprecated value to the `deprecated` dict in
+`cirq-<module>/cirq/protocols/json_test_data/spec.py`, of the form: 
+```python
+deprecated={
+ 'DeprecatedClass': 'deprecation_deadline',
+}
+```
+
 ## Removing a serializable value
 
 When a serializable value is removed from cirq, old serialized instances