fix(AIP-158): clarify degraded skip response guidance (#1510)

Author: noahdietz

aip/general/0158.md

@@ -110,9 +110,10 @@ For example:
   first 50 results were returned on the first page) and a `skip` value of `30`
   returns a single page of results starting with the 81st result.
 
-If a `skip` value is provided that causes the cursor to move past the end of
-the collection of results, the response **must** be `200 OK` with an empty
-result set, and not provide a `next_page_token`.
+If a `skip` value is provided that cannot be fulfilled e.g. due to latency of
+querying a massive data set, the response **must** be `200 OK` with an empty
+result set. If it is _known_ to put the cursor beyond the total size of the
+collection, the response **must not** include a `next_page_token`.
 
 ### Opacity
 
@@ -176,8 +177,23 @@ paginate) is reasonable for initially-small collections.
 
 [list]: ./0132.md
 
+## Rationale
+
+### Degraded `skip` response
+
+Large collections, complex queries, and globally distributed data can all
+contribute to a paginated method being unable to quickly or confidently fulfill
+a given `skip` request. Backend queries can timeout, data collation can take
+time, and the end user experience need not suffer as a result. In such cases,
+the pagination interface can be leveraged to keep the client engaged by
+providing a `next_page_token`, while the service collects an appropriate result.
+When the service has definitively determined that the requested `skip` exceeds
+the available results, the pagination interface is again applied and
+`next_page_token` is omitted to signal the end of results.
+
 ## Changelog
 
+- **2025-07-08**: Clarify degraded skip response guidance
 - **2020-05-24**: Clarified that adding pagination breaks client
   libraries.
 - **2020-05-13**: Added guidance for skipping results.