Update userdocs for file status icons

[emteknetnz]

Relates to work done in silverstripe/silverstripe-asset-admin#1066

and should cover both the two 'userform submission' icons and the 'restricted access' icons.

ACs (draft)

• The meaning of the new icons are understood
• It is clear how updating the file permissions will alter the icons
• There is a clear connection within the guide on file permissions as a broader topic (this isn't only userforms related)
• There is background provided on why these icons have been created, and what they aim to help with, i.e stronger exposure to the potential risk of not realising restricted files are being exposed to the public/ website visitors
• All areas where these icons can be seen are noted

PRs

• DOCS Add user help for securing file upload form submissions #980
• DOCS Add new user help for file permissions and secure files/folders silverstripe-userhelp-content#146

[sachajudd]

This will rely on #960 being merged and also preferably #970 in the near future but isn't a blocker.

[brynwhyman]

I've added some draft ACs to help set the scope

[clarkepaul]

This is currently in progress by the Silverstripe UX team.

[brynwhyman]

Ready for peer review :) added PRs to the description

[sachajudd]

Updated and reassigned to @bergice