arrow

GH-35601: [R][Documentation] Add missing docs to fileysystem.R (#35895)

### What changes are included in this PR?

Just new docstrings to cover missing ones.

### Are these changes tested?

I built the package locally and previewed the new help pages.

### Are there any user-facing changes?

No

* Closes: #35601

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>