Update the resource revisions guide using nested collections (take 2) (#1192)

Moving resource revisions to a collection-oriented pattern, reducing exceptional situations (see rationale in diff for more information).
---------

Co-authored-by: Difan Chen <[email protected]>

Author: toumorokoshi

aip/general/0122.md

@@ -282,11 +282,6 @@ When a field represents another resource, the field **should** be of type
 `string` and accept the resource name of the other resource. The field name
 **should** be equivalent to the corresponding message's name in snake case.
 
-The field **should not** be of type `message` using the `message` that
-implements the resource, _unless_ the API is internal-only, has tight lifecycle
-relationships, and has a permission model that enables inherited access to
-embedded resources.
-
 - Field names **may** include a leading adjective if appropriate (such as
   `string dusty_book`).
 - Field names **should not** use the `_name` suffix unless the field would be
@@ -297,6 +292,16 @@ embedded resources.
 - If using the resource name is not possible and using the ID component alone is
 strictly necessary, the field **should** use an `_id` suffix (e.g. `shelf_id`).
 
+The field **should not** be of type `message` using the `message` that
+implements the resource, __except__ for one of following conditions:
+
+- The API is internal-only, has tight lifecycle relationships, and has a
+  permission model that enables inherited access to embedded resources.
+- The embedding of the resource is done as part of the AIP-162 revisions
+  pattern.
+
+Example of a resource reference:
+
 ```proto
 // A representation of a book in a library.
 message Book {
@@ -349,6 +354,8 @@ isolation of logical concerns per-resource.
 
 ## Changelog
 
+- **2023-09-01**: Add a clause that allows embedding for revision resource
+  messages.
 - **2023-08-10**: Explicitly disallow embedding resource messages in a resource.
 - **2023-03-24**: Correction: full resource name contains the service name rather
   than the service endpoint