Address review feedback

Author: JoelSpeed

contributors/devel/sig-architecture/api-conventions.md

@@ -871,13 +871,21 @@ have a built-in `nil` value.
 
 The `+nullable` comment tag allows the json `null` value to be a valid value
 for a field. The `null` value is serialized only when a field is a pointer
-in the Go definition, and does not have the `omitempty` json tag.
+in the Go definition, and does not have the `omitempty` json tag, or sometimes where a
+custom marshal function is implemented.
 
 For example, a map field marked with `+nullable` would accept either `foo: null` or `foo: {}`.
 
 Usage of `+nullable` is discouraged as it introduces several issues:
 - It is not compatible with json merge patching.
-- It is not compatible with generic protobuf.
+  - From the [JSON Merge Patch RFC](https://tools.ietf.org/html/rfc7386#section-1):
+    > Null values in the merge patch are given special meaning to indicate the removal of existing values in the target.
+- Explicit `null` values are not be persisted in proto serializations.
+- `null` values are not supported by Server-Side Apply applyconfiguration types.
+  - A persisted `null` value would not round-trip through the applyconfiguration type
+    encode/decode cycle.
+
+Avoid designing APIs that require the distinction bewteen unset and `null`.
 
 ## Defaulting