Reference how Bulk Edit handles required fields

Currently, there's no public documentation that explains how Bulk Edit handles required fields. This could be referenced in the existing how-to "Create a Bulk Edit Form Using Nuxeo Studio" (could be a small note).

Details can be found on the acceptance criteria of WEBUI-380:

When a property is required:

• Widget should not mark it as required by default (reason is we can't tell if a document you selected actually holds that property)
• The "remove value(s)" option is disabled
• If I try to click on the option anyway, a notification is displayed: "This field requires a non empty value."