Docs: Clarify multi-select behavior in collection and gallery add controllers

[yogeshbhutkar]

Description

Updated the JSDoc documentation for the multiple attribute in GalleryAdd and CollectionAdd media controllers to accurately reflect its behavior with different values.

Context

The multiple attribute accepts both boolean and string values, with different behaviors:

• multiple=true: Requires SHIFT/CMD/CTRL key to select multiple items
• multiple='add': Allows selecting multiple items by clicking thumbnails directly

Technical Changes

• Updated JSDoc type annotation to {boolean|string} in both controllers
• Added detailed description of behavior differences between true and 'add' values
• Removed outdated TODO comment that incorrectly suggested 'add' had no special behavior

Files Changed

• src/js/media/controllers/gallery-add.js
• src/js/media/controllers/collection-add.js

Trac ticket: https://core.trac.wordpress.org/ticket/62696

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