[R][Docs] Update documentation about macOS build system

[assignUser]

Describe the enhancement requested

#37923 overhauls the macOS build system. We should make sure that the documentation reflects the changes once we honed in on the right default behavior and other details. This should include function docstrings (install-arrow) and similar.

Component(s)

Documentation, R

[jonkeane]

This is definitely a blocker for 14.0.0 yeah?