Add meaningful description to the User Fields dialog

[AlbyIanna]

Describe the request

Add a meaningful description to the User Fields dialog in order to better communicate the purpose of it.

Description to be added before the input fields:

Additional configuration parameters can be set for this board before uploading.

Describe the current behavior

Currently, the dialog doesn't provide any description, it just shows a list of input fields with labels:

Arduino IDE version

2.0.1

Operating system

macOS

Operating system version

12.3.1 (21E258)

Additional context

No response

Issue checklist

• I searched for previous requests in the issue tracker
• I verified the feature was still missing when using the latest nightly build
• My request contains all necessary details