Enhance Preferences Roles UX

Based on comments on https://github.com/solid/userguide/issues/17.

User Story

As a Data Browser user, I want to clearly understand the use of Roles within the Data Browser and how to select them, so I can access advanced Data Browser functionality.

Acceptance Criteria

1. The Roles section of the Preferences page displays:
   a. A short description about Roles and their use within the Data Browser.
   b. A tip on how to select multiple roles.
2. The Userguide is updated to reflect the enhancement.

Dev Notes

• Use fieldset to group the roles with legend "Roles".
• Potentially use input=checkbox + label=for (instead of option) per role. Easy to select a role at a time, and for multiple role selections, it doesn't require key or click combinations.
• Reuse text from userguide#roles for the short description of a role either after each label or have a title (for tooltip) or some other indicator to help find out about the capabilities.

Start with the Preferences page's Roles section and the Userguide's roles content referenced in the issue. Review the existing role-selection UI and related guidance before making changes. Done means the Roles section has a description, multi-selection tip, and the Userguide reflects the enhancement.