message: Clarify that looping forever is an acceptable process behavior.
diff: @@ -48,9 +48,9 @@ user-provided `main` function.
  At this point, the user's `main` starts executing, and `libtock_runtime` is no
  longer in control. Note that unlike most Rust programs, `main` is expected to
  -*not* return. Process binaries should use the `exit` system call to terminate
  -when they are done executing (which may be done as the last statement of
  -`main`).
  +*not* return. Process binaries can loop forever or use the `exit` system call to
  +terminate when they are done executing (which may be done as the last statement
  +of `main`).
  ## Appendix: Why `#![no_main]`?
  