Clarify @Nullable and @Nonnull usage [ci skip]

doc/dev/decisionrecords/Codestyle.md

@@ -158,6 +158,21 @@ What to put in Javadoc:
     - Is it immutable or should anything be treated as immutable
     - Is it a utility class of static methods that should not be instantiated
 
+### Annotations
+
+- On methods:
+    - Method should be marked as `@Nullable` if they can return null values
+    - Method parameters should be marked as `@Nullable`.
+- On fields:
+    - Fields should often be marked as `@Nullable` if they are nullable. If the class
+    only exposes the field through a simple getter method, using the annotation on field is optional,
+    but on the getter method it is required.
+
+Use of `@Nonnull` annotation is not allowed. It should be assumed methods/parameters/fields
+are non-null if they are not marked as `@Nullable`. However, there are places where the
+`@Nullable` annotation is missing even if it should have been used. Those can be updated
+to use the `@Nullable` annotation.
+
 ## JavaScript
 
 As of #206, we

src/main/java/org/opentripplanner/transit/model/package.md

@@ -23,10 +23,10 @@ All transit entities must have an ID. Transit entities ar "root" level are consi
 roots_.
 
 
-#### @Nonnull and @Nullable entity fields
+#### Non-null and nullable entity fields
 
-All fields getters(except primitive types) should be annotated with `@Nullable` or `@Nonnull`. None 
-null field should be enforced in the Entity constructor by using `Objects.requireNonNull`, 
+All fields getters(except primitive types) should be annotated with `@Nullable` if they can return null.
+Non-nullability of fields should be enforced in the Entity constructor by using `Objects.requireNonNull`,
 `Objects.requireNonNullElse` or `ObjectUtils.ifNotNull`. We should enforce this for all fields 
 required in both GTFS and in the Nordic NeTEx Profile. For enumeration types using a special value
 like `UNKNOWN` is preferred over making the field optional.