Clarification of BFCache handling

[simon-friedberger]

This seems only tangentially related to the rest of the document so maybe it deserves a more detailed explanation:

Interaction with Inactive Documents (BFCache, Prerendering)

When a session is ended for any reason, any inactive documents which had access to that session's credentials should be destroyed. This ensures that pages in BFCache or that are pre-rendering that contain information guarded by those credentials are not presented after the session has ended.