fix(AIP-121): clarify stateless protocol definition (#1184)

Author: noahdietz

aip/general/0121.md

@@ -97,8 +97,12 @@ patterns, such as database transactions, import and export, or data analysis.
 ### Stateless protocol
 
 As with most public APIs available today, resource-oriented APIs **must**
-operate over a stateless protocol: The fundamental behavior of any individual
-request is independent of other requests made by the caller.
+operate over a [stateless protocol][]: The fundamental behavior of any
+individual request is independent of other requests made by the caller.
+This is to say, each request happens in isolation of other requests made by that
+client or another, and resources exposed by an API are directly addressable
+without needing to apply a series of specific requests to "reach" the desired
+resource.
 
 In an API with a stateless protocol, the server has the responsibility for
 persisting data, which may be shared between multiple clients, while clients
@@ -127,6 +131,7 @@ and in turn do not increase resource management complexity.
 
 [rest]: https://en.wikipedia.org/wiki/Representational_state_transfer
 [rpc]: https://en.wikipedia.org/wiki/Remote_procedure_call
+[stateless protocol]: https://en.wikipedia.org/wiki/Stateless_protocol
 [get]: ./0131.md
 [list]: ./0132.md
 [create]: ./0133.md
@@ -141,6 +146,7 @@ and in turn do not increase resource management complexity.
 
 ## Changelog
 
+- **2023-07-23**: Clarify stateless protocol definition.
 - **2023-01-21**: Explicitly require matching schema across standard methods.
 - **2022-12-19**: Added a section requiring Get and List.
 - **2022-11-02**: Added a section restricting resource references.