[AIP-134] Add rationale on update_mask being optional

Add text explaining why `update_mask` must be OPTIONAL over REQUIRED.

aip/general/0134.md

@@ -103,6 +103,9 @@ message UpdateBookRequest {
   - The field **must** be optional, and the service **must** treat an omitted
     field mask as an implied field mask equivalent to all fields that are
     populated (have a non-empty value).
+    - Rationale is that omission of `update_mask` is equivalent on it being
+      empty. Marking this field as `REQUIRED` provides no benefit to the API
+      and results in an inconsistent experience across services.
   - Update masks **must** support a special value `*`, meaning full replacement
     (the equivalent of `PUT`).
 - The request message **must not** contain any other required fields, and