PnetCDF docs update

[adammoody]

Highlight need to call ncmpi_sync between variable definition, fill operations, and write calls.

Description

Motivation and Context

How Has This Been 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.
• [ ] 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.