docs: update 6-10-revenue-schedules.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added a note in the "Applying a Schedule on a Sales Invoice" section explaining that when no explicit start date is set, the release schedule defaults to beginning on the document's posting date (the accounting-relevant date) rather than the document date.

Source PRs

• light#7277 — feat(BLUE-1094): base release schedule start on posting date

How to use this PR

1. Review the diff against the existing article — focus on accuracy and customer voice.
2. Merge if it reads well. Edit and push to this branch if it needs adjustment.
3. Close without merging if the article doesn't actually need this update.

Once we have a track record of clean drafts, we'll graduate to auto-merge with a delay buffer. For now: human-in-the-loop.