fix(AIP-156): Do not define Update on singletons with only output only fields. (#1188)

Clarifying that singletons with only OUTPUT_ONLY fields do not require an Update method.

Author: lukesneeringer

aip/general/0156.md

@@ -20,6 +20,7 @@ always exist by virtue of the existence of its parent, with one and exactly one
 per parent.
 
 For example:
+
 ```proto
 message Config {
   option (google.api.resource) = {
@@ -32,6 +33,7 @@ message Config {
 ```
 
 The `Config` singleton would have the following RPCs:
+
 ```proto
 rpc GetConfig(GetConfigRequest) returns (Config) {
   option (google.api.http) = {
@@ -57,12 +59,15 @@ rpc UpdateConfig(UpdateConfigRequest) returns (Config) {
 - Singleton resources **should** define the [`Get`][aip-131] and
   [`Update`][aip-134] methods, and **may** define custom methods as
   appropriate.
+  - However, singleton resources **must not** define the [`Update`][aip-134]
+    method if all fields on the resource are [output only][aip-203].
 - Singleton resources are always singular.
-  - Example: 'users/1234/thing'
+  - Example: `users/1234/thing`
 - Singleton resources **may** parent other resources.
 
 ## Changelog
 
+- **2023-07-26:** Clarified that read-only singletons should not have `Update`.
 - **2021-11-02:** Added an example message and state parent eligibility.
 - **2021-01-14:** Changed example from `settings` to `config` for clarity.
 
@@ -72,3 +77,4 @@ rpc UpdateConfig(UpdateConfigRequest) returns (Config) {
 [aip-133]: ./0133.md
 [aip-134]: ./0134.md
 [aip-135]: ./0135.md
+[aip-203]: ./0203.md#output-only