Upload: Add admonition that uploading massive numbers of files simultaneously will degrade performance

[rolfsmeds]

Although the Upload component doesn't have a specific limit to the number of files that can be uploaded simultaneously, UI performance will typically degrade noticeably past a few hundred files, and, depending on the browser, XHR limitations may also break the process if more than 2000+ files are uploaded in one go.

This should be noted on the Upload docs page with an admonition.