api returns: review pass-thrus (#263)

Step 3f for #197 (comment)

- `create-dirs` update test to match spirit of vague path return type
- `set-attribute` document that `path` is return, adjust tests
- `set-creation-time` document that `path` is return, adjust tests
- `set-last-modified-time` document that `path` is return, adjust tests
- `set-posix-file-permissions` document that `path` is return, adjust tests
- `write-bytes` document that `file` is return, adjust tests
- `write-lines` document that `file` is return, adjust tests

Author: lread