[Nix Docs] Update macOS support status

Summary:
After being able to successfully follow each of the development
workflows, this should be repeatable by other developers.

Part of https://linear.app/comm/issue/ENG-1940

Depends on D6082

Test Plan:
N/A, however, if you did want to:

• Reset the entire mac.
• Follow development steps starting from https://github.com/CommE2E/comm/blob/jonringer/order-initial-nix-docs/docs/nix_dev_env.md#prerequisites

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6083