Add example for ABSL_FLAG with proto messages.

The example shows how to declare an ABSL_FLAG using a proto message type.

PiperOrigin-RevId: 872933272

Author: protobuf-github-bot

content/reference/cpp/cpp-generated.md

@@ -174,6 +174,13 @@ The valid options are:
 Note that flag support for message types is not provided for `LITE_RUNTIME`
 configurations.
 
+Example:
+
+```cpp
+ABSL_FLAG(MyProtoType, my_proto_config, {},
+          "This is a proto config description.");
+```
+
 ### Nested Types {#nested-types}
 
 A message can be declared inside another message. For example: