Clarify BFT validator requirements

[rolandtyler]

Pull request checklist

Use the following template to make sure your PR fits the GoQuorum documentation standard.

Before creating the PR

Make sure that:

• You've read the contribution guidelines.
• You've previewed your changes locally.

Describe the change

Clarify BFT validator requirements

Issue fixed

Fixes #237

Impacted parts

For content changes:

• Documentation content
• Documentation page organization

For tool changes:

• Build and QA tools (e.g., lint)
• MkDocs templates
• MkDocs configuration
• Python dependencies
• Node dependencies and JavaScript
• GitHub integration

After creating your PR and tests have finished

Make sure that:

• You've fixed any issues raised by the tests.
• You've previewed your changes on GitHub Pages

[alexandratran]

LGTM.