docs: Explain the open_dialog builder and footer contracts

[xxww0098]

Description

Creating input entities inside an open_dialog builder resets editing state when the dialog layer rebuilds. Focusing content unconditionally in that builder can also steal focus from other controls or nested dialogs. These contracts were unclear while building SkillStar.

Document the builder contract for open_dialog, open_alert_dialog, open_sheet, and open_sheet_at:

• Builders are render closures that may run again when their layer rebuilds. View caching means they are not guaranteed to run on every displayed frame.
• Create persistent entities before opening and capture their handles to preserve editing state.
• open_dialog initially focuses the dialog shell. Set initial content focus only on the first builder invocation, using a separate one-time guard for each opening.

The API docs include an input example with a Cell guard, and the English and Chinese Dialog pages explain the same lifecycle and focus rules.

Clarify the existing footer contract: plain Dialog does not generate buttons from button_props; those props install action callbacks. Applications compose their own DialogFooter. AlertDialog generates a default footer when no custom footer is supplied. Correct the private header setter's copied footer description as well.

Documentation only; no public signatures or runtime behavior change.

Validation

• cargo fmt --all --check and git diff --check — passed.
• cargo doc -p gpui-component --no-deps — passed; warnings remain at unmodified documentation locations.
• Runtime tests were not rerun for this documentation-only revision.

The documentation revision was generated with AI assistance.