Amend File API design doc to explicit open/close functions scope

docs/design/019-file-api.md

@@ -189,10 +189,18 @@ function ab2str(buf) {
 
 The proposed API is made feasible through the following implementation aspects:
 
--   A file's content is loaded into memory only once:
+- A file's content is loaded into memory only once:
     -   When a file is opened, its content is buffered at the module's root in a dedicated registry, returning a handle with a pointer to that buffer.
     -   Each VU receives a copy of the file handle, enabling them to interact with files using the unique memory area linked to the handle, instead of each receiving a full copy of the buffer.
     - As each invocation of `open*` receives a unique file handle linked to the same memory area, they each have unique offsets. This setup allows each VU to process file data independently without conflict or race conditions.
+- Scoped file closing:
+    - Based on the current implementation of k6, file opening is confined to the init context.
+    - Given this, we operate under the presumption that the lifespan of a file handle aligns with that of the init context. Furthermore, we recognize the possibility that the init context may undergo multiple evaluations throughout a script's execution.
+    - As a result, the `File` interface's `close` method is similarly bound to the init context's scope. That is, it can either be invoked outside the VU stage (in the `handleSummary` function for instance) or be automatically executed, implicitly, at the conclusion of the test, harnessing [the k6 internal event system](https://github.com/grafana/k6/pull/3112).
+    - Future refinements:
+      - In subsequent versions, [once we enable file opening within the VU context](https://github.com/grafana/k6/issues/3020), we would then permit and advocate for files to be closed within the VU stage as well.
+      - Presuming the design of the open function remains consistent with the current proposal, our underlying mechanism would pivot to a reference counting system. This would ensure that a file is only closed when all VUs have concluded their interactions with it.
+
 
 ### Possible future improvements