Add some detail to milestone #1

This change adds more information about what needs to be done
to implement `/dev/fuse`

FUTURE_COPYBARA_INTEGRATE_REVIEW=#2855 from ridwanmsharif:ridwanmsharif/fuse-doc-edit 5173c96
PiperOrigin-RevId: 314428000

Author: ridwanmsharif

pkg/sentry/fs/g3doc/fuse.md

@@ -76,7 +76,8 @@ ops can be implemented in parallel.
 
 #### Minimal client that can mount a trivial FUSE filesystem.
 
--   Implement `/dev/fuse`.
+-   Implement `/dev/fuse` - a character device used to establish an FD for
+    communication between the sentry and the server daemon.
 
 -   Implement basic FUSE ops like `FUSE_INIT`, `FUSE_DESTROY`.