Update UnifyFS high level description

[kathrynmohror]

Update high level description

Description

Remove references to "burst buffers"

Motivation and Context

The term "burst buffer is dated"

How Has This Been Tested?

Not tested

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Performance enhancement (non-breaking change which improves efficiency)
• [ ] Code cleanup (non-breaking change which makes code smaller or more readable)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Testing (addition of new tests or update to current tests)
• [x ] Documentation (a change to man pages or other documentation)

Checklist:

• [ ] My code follows the UnifyFS code style requirements.
• [x ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [ ] All commit messages are properly formatted.