Address review feedback

Author: JoelSpeed

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

@@ -877,7 +877,9 @@ For example, a map field marked with `+nullable` would accept either `foo: null`
 
 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
 
 ## Defaulting