docs(util): improve JSDoc for parseSelector, reflow and getNextActiveElement

[fauzan171]

Description

This PR improves the JSDoc blocks for three public utility helpers exported from js/src/util/index.js. No runtime behaviour is changed — only comments.

parseSelector

• Rewrote the description to be more explicit about why escaping is needed (special characters such as / and . in IDs break document.querySelector).
• Added descriptions for @param and @returns.

reflow

• Clarified the description ("by forcing a reflow").
• Documented the @param and added @returns {void}.
• Replaced the non-standard @return void with the canonical @returns {void} for consistency with the rest of the file (e.g. parseSelector).

getNextActiveElement

• Added the missing type annotations for activeElement, shouldGetNext and isCycleAllowed (previously only list had a type).
• Added a description for isCycleAllowed, which previously had none.
• Fixed the return type: {Element|elem} was incorrect (elem is not a type). It is now {Element}.
• Switched @return to @returns to match the rest of the file.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor
• Docs / typos
• New feature

Checklist

• My code follows the code style of this project.
• My change generates no new warnings.
• I have read the contributing guidelines.

Why

These three helpers are the only ones in util/index.js that carry JSDoc blocks, but the blocks were inconsistent (mixed @return/@returns, missing types, missing descriptions, an invalid return type). Cleaning them up improves IDE hover hints, TypeDoc/VSCode IntelliSense and makes the intent of the helpers easier to grok at a glance.