Add a note about massive numbers of files causing degradation in performance

[HerbertsVaadin]

Solves #4610

[github-actions]

AI Language Review

In the file "upload.adoc", the only modification observed was the addition of a new note under the section "File Count" with information about potential performance degradation. This note advises setting a file count limit to prevent performance issues and mentions browser-specific XHR limitations. This addition is clear and informative, providing valuable guidance to users. There are no issues that require improvement in this modification.