Document XBPS cache-bypass refresh

docs/spec/xbps-proof-results.md

@@ -68,8 +68,8 @@ Cache behavior:
 - Conditional requests with old ETag return 200 (full content), not 304
 
 xbps-install behavior:
-- Always fetches fresh repodata with `-S` flag
-- Respects ETag changes for immediate discovery
+- `-M -S` fetches fresh repodata while bypassing the on-disk cache
+- The ordinary `-S` path can reuse a cached repodata archive
 - No 6-hour delay observed in practice
 
 Binary size limits:
@@ -77,8 +77,9 @@ Binary size limits:
 - Real packages expected to be <10MB (vendored pure-Python)
 - Gitea serves any file size without LFS
 
-**Caveat**: Clients using `xbps-install -Su` without `-S` may use cached repodata.
-The `-S` flag forces a fresh fetch. Users should always use `-Syu` for updates.
+**Caveat**: Clients using `xbps-install -Su` or `-Syu` without `-M` may use
+cached repodata. Users should use `-M -Syu` for updates when immediate
+publication visibility matters.
 
 ### 5. Publish index/artifacts together, preserve older artifacts, safe failure recovery