addressing feedback.

aep/general/0162/aep.md.j2

@@ -48,7 +48,7 @@ message BookRevision {
     [(google.api.field_behavior) = OUTPUT_ONLY];
 
   // Other revision IDs that share the same snapshot.
-  repeated string alternate_ids = 4
+  repeated string aliases = 4
     [(google.api.field_behavior) = OUTPUT_ONLY];
 }
 ```
@@ -129,6 +129,8 @@ Services **may** reserve specific IDs to be [aliases][alias] (e.g.
 GET /v1/publishers/{publisher_id}/books/{book_id}/revisions/{revision_id}
 ```
 
+If specific IDs are reserved, services **should** document those IDs.
+
 - If a `latest` ID exists, it **must** represent the most recently created
 revision. The content of `publishers/{publisher}/books/{book}/revisions/latest`
 and `publishers/{publisher}/books/{book}` can differ, as the latest revision may