fs-mistrust: commit to stable env var handling

Since these environment variables (for example ARTI_FS_DISABLE_PERMISSION_CHECKS) become part of the stable API for applications that use fs-mistrust, it should be explicit about how these environment variables are interpreted so that applications can show the values in their documentation or help text.

It would be nice to have unit tests for these, but there's no good way to test environment variables.

Edit: This MR originally added a new public falsy_env_var_values function, but this was reverted and instead only documentation changes are made.