Update uploader.md onBeforeUpload event

[Lukortech]

Description

I've found some information scattered around the web about this UI element but the documentation doesn't mention this event and even more intruiguing is that it doesn't really work as I would expect it to.

I should be able to validate files (like exceeded size, fileType) before even allowing user to click "upload" button. That's not the case and I've already got davin to write some code:

https://github.com/box/box-ui-elements/pull/3841

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own changes
• [ ] I have run yarn lint to make sure my changes pass all linters
• [x] I have pulled the latest changes from the upstream developer branch

[CLAassistant]

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