AIP-180: Clarify scope of compatibility rules. (#1187)

Author: jskeet

aip/general/0180.md

@@ -44,6 +44,13 @@ There are three distinct types of compatibility to consider:
 buffers and JSON as transport formats. Other transport formats may have
 slightly different rules.
 
+**Note:** This guidance assumes that APIs are intended to be called from a
+range of consumers, written in multiple languages and with no control over
+how and when consumers update. Any API which has a more limited scope (for
+example, an API which is only called by client code written by the same team
+as the API producer, or deployed in a way which can enforce updates) should
+carefully consider its own compatibility requirements.
+
 ### Adding components
 
 In general, new components (interfaces, methods, messages, fields, enums, or
@@ -236,6 +243,7 @@ version.
 
 ## Changelog
 
+- **2023-07-26**: Added note on APIs which have limited clients.
 - **2022-08-11**: Added "Moving components between files" section.
 - **2022-06-01**: Added more links to other AIPs with compatibility concerns
 - **2019-12-16**: Clarified that moving existing fields into oneofs is