fix(update): add json-merge-patch guidance for HTTP

After offline discussion, it was agreed upon that IETF RFC 7396 (JSON Merge Patch) is the more sensible choice for a partial update on an HTTP+JSON API. The rationale includes:

- can generally be expressed via protobuf field masks.
- aligns with the AEPs usage of resource-oriented guidance.
- aligns with an existing IETF standard, which as already been widely adopted.

aep/general/0134/aep.md.j2

@@ -100,6 +100,12 @@ Update methods implement a common request pattern:
 
 {% sample '../example.oas.yaml', '$.paths./publishers.post.requestBody' %}
 
+- The method **must** adhere to the behavior specified in [IETF RFC 7396 - JSON
+  Merge Patch](https://datatracker.ietf.org/doc/html/rfc7396).
+- The method **must** support MIME types `application/merge-patch+json` to
+  adhere to IETF RFC 7396. The method **may** support `application/json` as a
+  common generalization.
+
 {% endtabs %}
 
 ### Responses

aep/general/example.oas.yaml

@@ -202,14 +202,14 @@ paths:
           type: string
       requestBody:
         content:
-          application/json:
+          application/merge-patch+json:
             schema:
               $ref: '#/components/schemas/publisher'
         required: true
       responses:
         '200':
           content:
-            application/json:
+            application/merge-patch+json:
               schema:
                 $ref: '#/components/schemas/publisher'
           description: Successful response
@@ -311,14 +311,14 @@ paths:
           type: string
       requestBody:
         content:
-          application/json:
+          application/merge-patch+json:
             schema:
               $ref: '#/components/schemas/book'
         required: true
       responses:
         '200':
           content:
-            application/json:
+            application/merge-patch+json:
               schema:
                 $ref: '#/components/schemas/book'
           description: Successful response