docs(task): Clarify --due is a server-side pass-through

[engfragui]

Background

Adds documentation/clarification re: #342.

The --due value on task add / task update is sent verbatim as due_string to the Backend API (the CLI does not parse or rewrite it), i.e. does not handle starting <date> clauses as the user might expect: strings like every! 2 weeks starting 2026-05-17 end up stored as the entire literal recurrence rule and the task never advances on completion.

Reference

• Helps with Recurring task created via due_string with a starting clause never advances on completion #342

[doistbot]

Thank you for adding this helpful documentation to clarify the --due parsing limitations and steering users toward the natural language capabilities of quickadd. This update will significantly reduce user frustration around complex recurrence rules. A couple of adjustments were flagged during review, specifically regarding the need to scope the quickadd recommendation strictly to task creation rather than updates, as well as the need for test coverage of the help output in task.test.ts to ensure the new guidance is preserved.

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[engfragui]

@scottlovegrove Apologies, I keep forgetting we automatically add a review right away to PRs in this repo, so I guess it tagged you when I was still working on the PR (edit: it actually tagged Ernesto and you were just a lurker!). Either way, I'm done now (just did a little re-wording, nothing much).