docs(collections): explain updating collection structure

[patrickwehbe]

Closes

Closes #10199

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Docs-only change. Build the docs site and view the Collections page (react-aria/collections); the new "Updating the collection structure" subsection appears between "Dependencies" and "Combining collections".

🧢 Your Project:

React Aria docs

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This adds a focused subsection to the Collections guide explaining a behavior that has tripped up several users (see #10199): state that controls which items exist in a collection must live above the collection component.

Collections are built by parsing the JSX tree itself rather than the output of the render function. To do that, the component renders a hidden copy of its children up to each item boundary to build the collection document, and only then renders the real, interactive content. Because these are two separate renders, a state update inside the collection's children re-renders only the visible content and never re-runs the hidden structure pass, so toggling local state (e.g. from SelectStateContext inside a Select popover) cannot add or remove items. Lifting that state above the collection re-runs both passes.

Per @snowystinger's suggestion in the issue this lives in the Collections page since it applies to all collections, not just Select. The explanation adapts @nwidynski's write-up from the issue thread. Prose only; no API changes.