Adding a description to the sharepoint list

SebastianSzyroki You can use SharePoint JSON form formatting: Configure layout > Header option to add text on top of list form.

Example: 

{
    "elmType": "div",
    "attributes": {
        "class": "ms-fontColor-themePrimary"
    },
    "txtContent": "This is a test sample list description on list form"
}

Output: 

You can adjust the JSON and CSS styling as per your requirements.

Documentation: Configure the list form - custom header - follow steps given here to add JSON to list form

Check few form JSON samples available here for JSON styling inspiration: Form Formatting samples for SharePoint and Microsoft Lists 

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