When a new CLI flag is added (e.g. --output-file for usethis tool requirements.txt), docs/cli/reference.md must be updated to document it.

This was overlooked when adding --output-file in PR #1794 — the flag was implemented and tested but not documented.

What: Failing to update docs/cli/reference.md when introducing new options.
Where: docs/cli/reference.md; src/usethis/_ui/options.py; src/usethis/_ui/interface/.
Why: Users rely on the reference docs to discover available flags. Undocumented flags effectively don't exist from the user's perspective.