Runs export: a truncated one counts the selection into X-Total-Count; the SDK and fluksio export warn how many runs were left out

docs/code/api.md

@@ -143,7 +143,9 @@ row's `created_at` as `before` for stable pagination.
 Exports also accept `ids` and `until`, with `format=csv` or `jsonl`.
 The metrics export is a long table; the runs export is one row per run.
 Use `params` and `metrics` to choose columns, including dotted paths into
-record outputs.
+record outputs. An export covers at most the newest 10 000 runs. When a
+selection matches more, the response carries `X-Truncated` with that cap and
+`X-Total-Count` with the number of runs the selection matched.
 
 Import takes `{"runs": [...]}`, records completed results without executing
 code, and deduplicates by `external_id`. See