Amendment DefaultVote standard

[wojake]

Add note to set amendments as DefaultVote::no to give the end decision to validators, not solely to rippled contributors.

High Level Overview of Change

Context of Change

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [X] Documentation Updates
• [ ] Release