This commit documents my learnings after having encountered a failure
I reported in the rclone forum[0].
I may be a fool for having failed to understand the previous
documentation, but I am likely not the only fool to get snared by it.
This commit therefore adds details to clarify what the user must do in
order to allow `--check-access` to succeed.
While at it, I've also added some basic documentation for `--check-filename`.
[0]: https://forum.rclone.org/t/bisync-check-file-check-failed/35682
Bryan Kaplan