Document evidence context source unit excerpts

docs/operations.md

@@ -110,9 +110,9 @@ are defined in [Accepted Chunk Map](integrations/chunk-map.md). An optional
 [evidence context](integrations/evidence-context.md) contains source-unit text
 and metadata. It is not a cache or debug artifact: retain it with the output
 bundle only for as long as consumers need it, and apply source-content access
-controls to the entire bundle. Selected lanes may collectively cite most of a
-transcript, so a broad allowlist can make the evidence artifact nearly as
-sensitive and large as the source itself.
+controls to the entire bundle. Its selected source-unit excerpt may include
+every source unit once when coverage is broad or its configured window is
+large, so do not assume a byte or token reduction or reduced sensitivity.
 
 ## Chunk-Plan Cache